A Graphics and Computing GPUs Imagination is more

A Graphics and Computing GPUs Imagination is more
A
A
P
P
E
N
D
Imagination is more
important than
knowledge.
Albert Einstein
On Science, 1930s
I
X
Graphics and
Computing GPUs
John Nickolls
Director of Architecture
NVIDIA
David Kirk
Chief Scientist
NVIDIA
A.1
Introduction A-3
A.2
GPU System Architectures A-7
A.3
Programming GPUs A-12
A.4
Multithreaded Multiprocessor Architecture A-25
A.5
Parallel Memory System A-36
A.6
Floating-point Arithmetic A-41
A.7
Real Stuff: The NVIDIA GeForce 8800 A-46
A.8
Real Stuff: Mapping Applications to GPUs A-55
A.9
Fallacies and Pitfalls A-72
A.10
Concluding Remarks A-76
A.11
Historical Perspective and Further Reading A-77
A.1
Introduction
This appendix focuses on the GPU—the ubiquitous graphics processing unit
in every PC, laptop, desktop computer, and workstation. In its most basic form,
the GPU generates 2D and 3D graphics, images, and video that enable windowbased operating systems, graphical user interfaces, video games, visual imaging
applications, and video. The modern GPU that we describe here is a highly
parallel, highly multithreaded multiprocessor optimized for visual computing.
To provide real-time visual interaction with computed objects via graphics,
images, and video, the GPU has a unified graphics and computing architecture
that serves as both a programmable graphics processor and a scalable parallel
computing platform. PCs and game consoles combine a GPU with a CPU to form
heterogeneous systems.
A Brief History of GPU Evolution
Fifteen years ago, there was no such thing as a GPU. Graphics on a PC were
performed by a video graphics array (VGA) controller. A VGA controller was
simply a memory controller and display generator connected to some DRAM. In
the 1990s, semiconductor technology advanced sufficiently that more functions
could be added to the VGA controller. By 1997, VGA controllers were beginning
to incorporate some three-dimensional (3D) acceleration functions, including
graphics processing
unit (GPU) A processor
optimized for 2D and 3D
graphics, video, visual
computing, and display.
visual computing A mix
of graphics processing
and computing that lets
you visually interact with
computed objects via
graphics, images, and
video.
heterogeneous system
A system combining
different processor types.
A PC is a heterogeneous
CPU–GPU system.
A-4
Appendix A Graphics and Computing GPUs
hardware for triangle setup and rasterization (dicing triangles into individual
pixels) and texture mapping and shading (applying “decals” or patterns to pixels
and blending colors).
In 2000, the single chip graphics processor incorporated almost every detail
of the traditional high-end workstation graphics pipeline and therefore, deserved
a new name beyond VGA controller. The term GPU was coined to denote that
the graphics device had become a processor.
Over time, GPUs became more programmable, as programmable processors
replaced fixed function dedicated logic while maintaining the basic 3D graphics
pipeline organization. In addition, computations became more precise over time,
progressing from indexed arithmetic, to integer and fixed point, to single precision
floating-point, and recently to double precision floating-point. GPUs have become
massively parallel programmable processors with hundreds of cores and thousands
of threads.
Recently, processor instructions and memory hardware were added to support
general purpose programming languages, and a programming environment was
created to allow GPUs to be programmed using familiar languages, including C
and C++. This innovation makes a GPU a fully general-purpose, programmable,
manycore processor, albeit still with some special benefits and limitations.
GPU Graphics Trends
application programming
interface (API) A set of
function and data structure
definitions providing an
interface to a library of
functions.
GPUs and their associated drivers implement the OpenGL and DirectX models of
graphics processing. OpenGL is an open standard for 3D graphics programming
available for most computers. DirectX is a series of Microsoft multimedia programming interfaces, including Direct3D for 3D graphics. Since these application
programming interfaces (APIs) have well-defined behavior, it is possible to build
effective hardware acceleration of the graphics processing functions defined by the
APIs. This is one of the reasons (in addition to increasing device density) that new
GPUs are being developed every 12 to 18 months that double the performance of
the previous generation on existing applications.
Frequent doubling of GPU performance enables new applications that were
not previously possible. The intersection of graphics processing and parallel
computing invites a new paradigm for graphics, known as visual computing. It
replaces large sections of the traditional sequential hardware graphics pipeline
model with programmable elements for geometry, vertex, and pixel programs.
Visual computing in a modern GPU combines graphics processing and parallel
computing in novel ways that permit new graphics algorithms to be implemented,
and open the door to entirely new parallel processing applications on pervasive
high-performance GPUs.
Heterogeneous System
Although the GPU is arguably the most parallel and most powerful processor in
a typical PC, it is certainly not the only processor. The CPU, now multicore and
A.1
Introduction
A-5
soon to be manycore, is a complementary, primarily serial processor companion
to the massively parallel manycore GPU. Together, these two types of processors
comprise a heterogeneous multiprocessor system.
The best performance for many applications comes from using both the CPU
and the GPU. This appendix will help you understand how and when to best split
the work between these two increasingly parallel processors.
GPU Evolves into Scalable Parallel Processor
GPUs have evolved functionally from hardwired, limited capability VGA controllers
to programmable parallel processors. This evolution has proceeded by changing
the logical (API-based) graphics pipeline to incorporate programmable elements
and also by making the underlying hardware pipeline stages less specialized and
more programmable. Eventually, it made sense to merge disparate programmable
pipeline elements into one unified array of many programmable processors.
In the GeForce 8-series generation of GPUs, the geometry, vertex, and pixel
processing all run on the same type of processor. This unification allows for
dramatic scalability. More programmable processor cores increase the total system
throughput. Unifying the processors also delivers very effective load balancing,
since any processing function can use the whole processor array. At the other end
of the spectrum, a processor array can now be built with very few processors, since
all of the functions can be run on the same processors.
Why CUDA and GPU Computing?
This uniform and scalable array of processors invites a new model of programming
for the GPU. The large amount of floating-point processing power in the GPU
processor array is very attractive for solving nongraphics problems. Given the large
degree of parallelism and the range of scalability of the processor array for graphics
applications, the programming model for more general computing must express
the massive parallelism directly, but allow for scalable execution.
GPU computing is the term coined for using the GPU for computing via a
parallel programming language and API, without using the traditional graphics
API and graphics pipeline model. This is in contrast to the earlier General Purpose
computation on GPU (GPGPU) approach, which involves programming the GPU
using a graphics API and graphics pipeline to perform nongraphics tasks.
Compute Unified Device Architecture (CUDA) is a scalable parallel programming model and software platform for the GPU and other parallel processors that
allows the programmer to bypass the graphics API and graphics interfaces of the
GPU and simply program in C or C++. The CUDA programming model has an
SPMD (single-program multiple data) software style, in which a programmer
writes a program for one thread that is instanced and executed by many threads
in parallel on the multiple processors of the GPU. In fact, CUDA also provides a
facility for programming multiple CPU cores as well, so CUDA is an environment
for writing parallel programs for the entire heterogeneous computer system.
GPU computing Using
a GPU for computing via
a parallel programming
language and API.
GPGPU Using a GPU
for general-purpose
computation via a
traditional graphics API
and graphics pipeline.
CUDA A scalable parallel
programming model
and language based on
C/C++. It is a parallel
programming platform
for GPUs and multicore
CPUs.
A-6
Appendix A Graphics and Computing GPUs
GPU Unifies Graphics and Computing
With the addition of CUDA and GPU computing to the capabilities of the GPU,
it is now possible to use the GPU as both a graphics processor and a computing
processor at the same time, and to combine these uses in visual computing
applications. The underlying processor architecture of the GPU is exposed in two
ways: first, as implementing the programmable graphics APIs, and second, as a
massively parallel processor array programmable in C/C++ with CUDA.
Although the underlying processors of the GPU are unified, it is not necessary
that all of the SPMD thread programs are the same. The GPU can run graphics
shader programs for the graphics aspect of the GPU, processing geometry, vertices,
and pixels, and also run thread programs in CUDA.
The GPU is truly a versatile multiprocessor architecture, supporting a variety of
processing tasks. GPUs are excellent at graphics and visual computing as they were
specifically designed for these applications. GPUs are also excellent at many generalpurpose throughput applications that are “first cousins” of graphics, in that they
perform a lot of parallel work, as well as having a lot of regular problem structure.
In general, they are a good match to data-parallel problems (see Chapter 7),
particularly large problems, but less so for less regular, smaller problems.
GPU Visual Computing Applications
Visual computing includes the traditional types of graphics applications plus many
new applications. The original purview of a GPU was “anything with pixels,” but it
now includes many problems without pixels but with regular computation and/or
data structure. GPUs are effective at 2D and 3D graphics, since that is the purpose
for which they are designed. Failure to deliver this application performance would
be fatal. 2D and 3D graphics use the GPU in its “graphics mode,” accessing the processing power of the GPU through the graphics APIs, OpenGLTM, and DirectXTM.
Games are built on the 3D graphics processing capability.
Beyond 2D and 3D graphics, image processing and video are important applications for GPUs. These can be implemented using the graphics APIs or as computational programs, using CUDA to program the GPU in computing mode. Using
CUDA, image processing is simply another data-parallel array program. To the
extent that the data access is regular and there is good locality, the program will
be efficient. In practice, image processing is a very good application for GPUs.
Video processing, especially encode and decode (compression and decompression
according to some standard algorithms) is quite efficient.
The greatest opportunity for visual computing applications on GPUs is to “break
the graphics pipeline.” Early GPUs implemented only specific graphics APIs, albeit
at very high performance. This was wonderful if the API supported the operations
that you wanted to do. If not, the GPU could not accelerate your task, because early
GPU functionality was immutable. Now, with the advent of GPU computing and
CUDA, these GPUs can be programmed to implement a different virtual pipeline
by simply writing a CUDA program to describe the computation and data flow
A.2
GPU System Architectures
that is desired. So, all applications are now possible, which will stimulate new visual
computing approaches.
A.2
GPU System Architectures
In this section, we survey GPU system architectures in common use today. We
discuss system configurations, GPU functions and services, standard programming
interfaces, and a basic GPU internal architecture.
Heterogeneous CPU–GPU System Architecture
A heterogeneous computer system architecture using a GPU and a CPU can be
described at a high level by two primary characteristics: first, how many functional
subsystems and/or chips are used and what are their interconnection technologies
and topology; and second, what memory subsystems are available to these functional
subsystems. See Chapter 6 for background on the PC I/O systems and chip sets.
The Historical PC (circa 1990)
Figure A.2.1 is a high-level block diagram of a legacy PC, circa 1990. The north
bridge (see Chapter 6) contains high-bandwidth interfaces, connecting the CPU,
memory, and PCI bus. The south bridge contains legacy interfaces and devices:
ISA bus (audio, LAN), interrupt controller; DMA controller; time/counter. In
this system, the display was driven by a simple framebuffer subsystem known
CPU
Front Side Bus
North
Bridge
Memory
PCI Bus
South
Bridge
LAN
FIGURE A.2.1
UART
VGA
Controller
Framebuffer
Memory
VGA
Display
Historical PC. VGA controller drives graphics display from framebuffer memory.
A-7
A-8
PCI-Express (PCIe)
A standard system I/O
interconnect that uses
point-to-point links.
Links have a configurable
number of lanes and
bandwidth.
Appendix A Graphics and Computing GPUs
as a VGA (video graphics array) which was attached to the PCI bus. Graphics
subsystems with built-in processing elements (GPUs) did not exist in the PC
landscape of 1990.
Figure A.2.2 illustrates two configurations in common use today. These are
characterized by a separate GPU (discrete GPU) and CPU with respective memory
subsystems. In Figure A.2.2a, with an Intel CPU, we see the GPU attached via a
16-lane PCI-Express 2.0 link to provide a peak 16 GB/s transfer rate, (peak of
8 GB/s in each direction). Similarly, in Figure A.2.2b, with an AMD CPU, the GPU
Intel
CPU
Front Side Bus
x16 PCI-Express Link
North
Bridge
GPU
display
x4 PCI-Express Link
derivative
GPU
Memory
DDR2
Memory
128-bit
667 MT/s
South
Bridge
(a)
AMD
CPU
CPU
core
128-bit
667 MT/s
internal bus
North
Bridge
x16 PCI-Express Link
GPU
DDR2
Memory
HyperTransport 1.03
Chipset
display
GPU
Memory
(b)
FIGURE A.2.2 Contemporary PCs with Intel and AMD CPUs. See Chapter 6 for an explanation of
the components and interconnects in this figure.
A.2
A-9
GPU System Architectures
is attached to the chipset, also via PCI-Express with the same available bandwidth.
In both cases, the GPUs and CPUs may access each other’s memory, albeit with
less available bandwidth than their access to the more directly attached memories.
In the case of the AMD system, the north bridge or memory controller is integrated
into the same die as the CPU.
A low-cost variation on these systems, a unified memory architecture (UMA)
system, uses only CPU system memory, omitting GPU memory from the system.
These systems have relatively low performance GPUs, since their achieved
performance is limited by the available system memory bandwidth and increased
latency of memory access, whereas dedicated GPU memory provides high
bandwidth and low latency.
A high performance system variation uses multiple attached GPUs, typically
two to four working in parallel, with their displays daisy-chained. An example is
the NVIDIA SLI (scalable link interconnect) multi-GPU system, designed for high
performance gaming and workstations.
The next system category integrates the GPU with the north bridge (Intel) or
chipset (AMD) with and without dedicated graphics memory.
Chapter 5 explains how caches maintain coherence in a shared address space.
With CPUs and GPUs, there are multiple address spaces. GPUs can access their
own physical local memory and the CPU system’s physical memory using virtual
addresses that are translated by an MMU on the GPU. The operating system kernel
manages the GPU’s page tables. A system physical page can be accessed using either
coherent or noncoherent PCI-Express transactions, determined by an attribute
in the GPU’s page table. The CPU can access GPU’s local memory through an
address range (also called aperture) in the PCI-Express address space.
unified memory
architecture (UMA)
A system architecture in
which the CPU and GPU
share a common system
memory.
Game Consoles
Console systems such as the Sony PlayStation 3 and the Microsoft Xbox 360
resemble the PC system architectures previously described. Console systems
are designed to be shipped with identical performance and functionality over
a lifespan that can last five years or more. During this time, a system may be
reimplemented many times to exploit more advanced silicon manufacturing
processes and thereby to provide constant capability at ever lower costs. Console
systems do not need to have their subsystems expanded and upgraded the way PC
systems do, so the major internal system buses tend to be customized rather than
standardized.
GPU Interfaces and Drivers
In a PC today, GPUs are attached to a CPU via PCI-Express. Earlier generations
used AGP. Graphics applications call OpenGL [Segal and Akeley, 2006] or
Direct3D [Microsoft DirectX Specification] API functions that use the GPU as
a coprocessor. The APIs send commands, programs, and data to the GPU via a
graphics device driver optimized for the particular GPU.
AGP An extended
version of the original PCI
I/O bus, which provided
up to eight times the
bandwidth of the original
PCI bus to a single card
slot. Its primary purpose
was to connect graphics
subsystems into PC
systems.
A-10
Appendix A Graphics and Computing GPUs
Graphics Logical Pipeline
The graphics logical pipeline is described in Section A.3. Figure A.2.3 illustrates
the major processing stages, and highlights the important programmable stages
(vertex, geometry, and pixel shader stages).
Input
Assembler
FIGURE A.2.3
Vertex
Shader
Geometry
Shader
Setup &
Rasterizer
Pixel
Shader
Raster Operations/
Output Merger
Graphics logical pipeline. Programmable graphics shader stages are blue, and fixed-function blocks are white.
Mapping Graphics Pipeline to Unified GPU Processors
Figure A.2.4 shows how the logical pipeline comprising separate independent
programmable stages is mapped onto a physical distributed array of processors.
Basic Unified GPU Architecture
Unified GPU architectures are based on a parallel array of many programmable
processors. They unify vertex, geometry, and pixel shader processing and parallel
computing on the same processors, unlike earlier GPUs which had separate
processors dedicated to each processing type. The programmable processor array is
tightly integrated with fixed function processors for texture filtering, rasterization,
raster operations, anti-aliasing, compression, decompression, display, video
decoding, and high-definition video processing. Although the fixed-function
processors significantly outperform more general programmable processors in
terms of absolute performance constrained by an area, cost, or power budget, we
will focus on the programmable processors here.
Compared with multicore CPUs, manycore GPUs have a different architectural
design point, one focused on executing many parallel threads efficiently on many
Input
Assembler
Vertex
Shader
Geometry
Shader
Setup &
Rasterizer
Pixel
Shader
Raster Operations/
Output Merger
Unified Processor
Array
FIGURE A.2.4 Logical pipeline mapped to physical processors. The programmable shader
stages execute on the array of unified processors, and the logical graphics pipeline dataflow recirculates
through the processors.
A.2
A-11
GPU System Architectures
processor cores. By using many simpler cores and optimizing for data-parallel
behavior among groups of threads, more of the per-chip transistor budget is
devoted to computation, and less to on-chip caches and overhead.
Processor Array
A unified GPU processor array contains many processor cores, typically organized
into multithreaded multiprocessors. Figure A.2.5 shows a GPU with an array of
112 streaming processor (SP) cores, organized as 14 multithreaded streaming
multiprocessors (SM). Each SP core is highly multithreaded, managing 96
concurrent threads and their state in hardware. The processors connect with
four 64-bit-wide DRAM partitions via an interconnection network. Each SM
has eight SP cores, two special function units (SFUs), instruction and constant
caches, a multithreaded instruction unit, and a shared memory. This is the basic
Tesla architecture implemented by the NVIDIA GeForce 8800. It has a unified
architecture in which the traditional graphics programs for vertex, geometry, and
pixel shading run on the unified SMs and their SP cores, and computing programs
run on the same processors.
Host CPU
System Memory
Bridge
GPU
Host Interface
Input Assembler
Viewport/Clip/
Setup/Raster/
ZCull
Vertex Work
Distribution
Pixel Work
Distribution
High-Definition
Video Processors
Compute Work
Distribution
SM
I-Cache
MT Issue
C-Cache
TPC
TPC
TPC
TPC
TPC
TPC
TPC
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
SP SP
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Texture Unit
Tex L1
Texture Unit
Tex L1
Texture Unit
Tex L1
Texture Unit
Tex L1
Texture Unit
Tex L1
Texture Unit
Tex L1
Interconnection Network
ROP
L2
DRAM
ROP
L2
DRAM
ROP
L2
DRAM
ROP
L2
DRAM
Display Interface
SP
SP
SP
SP
SP
SP
SP
SP
SFU
SFU
Shared
Memory
Display
FIGURE A.2.5 Basic unified GPU architecture. Example GPU with 112 streaming processor (SP) cores organized in 14 streaming
multiprocessors (SMs); the cores are highly multithreaded. It has the basic Tesla architecture of an NVIDIA GeForce 8800. The processors
connect with four 64-bit-wide DRAM partitions via an interconnection network. Each SM has eight SP cores, two special function units
(SFUs), instruction and constant caches, a multithreaded instruction unit, and a shared memory.
A-12
Appendix A Graphics and Computing GPUs
The processor array architecture is scalable to smaller and larger GPU configurations by scaling the number of multiprocessors and the number of memory
partitions. Figure A.2.5 shows seven clusters of two SMs sharing a texture unit and
a texture L1 cache. The texture unit delivers filtered results to the SM given a set of
coordinates into a texture map. Because filter regions of support often overlap for
successive texture requests, a small streaming L1 texture cache is effective to reduce
the number of requests to the memory system. The processor array connects with
raster operation (ROP) processors, L2 texture caches, external DRAM memories,
and system memory via a GPU-wide interconnection network. The number of
processors and number of memories can scale to design balanced GPU systems for
different performance and market segments.
A.3
Programming GPUs
Programming multiprocessor GPUs is qualitatively different than programming
other multiprocessors like multicore CPUs. GPUs provide two to three orders
of magnitude more thread and data parallelism than CPUs, scaling to hundreds
of processor cores and tens of thousands of concurrent threads in 2008. GPUs
continue to increase their parallelism, doubling it about every 12 to 18 months,
enabled by Moore’s law [1965] of increasing integrated circuit density and by
improving architectural efficiency. To span the wide price and performance range
of different market segments, different GPU products implement widely varying
numbers of processors and threads. Yet users expect games, graphics, imaging,
and computing applications to work on any GPU, regardless of how many parallel
threads it executes or how many parallel processor cores it has, and they expect
more expensive GPUs (with more threads and cores) to run applications faster.
As a result, GPU programming models and application programs are designed to
scale transparently to a wide range of parallelism.
The driving force behind the large number of parallel threads and cores in a
GPU is real-time graphics performance—the need to render complex 3D scenes
with high resolution at interactive frame rates, at least 60 frames per second.
Correspondingly, the scalable programming models of graphics shading languages
such as Cg (C for graphics) and HLSL (high-level shading language) are designed
to exploit large degrees of parallelism via many independent parallel threads and to
scale to any number of processor cores. The CUDA scalable parallel programming
model similarly enables general parallel computing applications to leverage large
numbers of parallel threads and scale to any number of parallel processor cores,
transparently to the application.
In these scalable programming models, the programmer writes code for a single
thread, and the GPU runs myriad thread instances in parallel. Programs thus scale
transparently over a wide range of hardware parallelism. This simple paradigm
arose from graphics APIs and shading languages that describe how to shade one
A.3
A-13
Programming GPUs
vertex or one pixel. It has remained an effective paradigm as GPUs have rapidly
increased their parallelism and performance since the late 1990s.
This section briefly describes programming GPUs for real-time graphics
applications using graphics APIs and programming languages. It then describes
programming GPUs for visual computing and general parallel computing
applications using the C language and the CUDA programming model.
Programming Real-Time Graphics
APIs have played an important role in the rapid, successful development of GPUs
and processors. There are two primary standard graphics APIs: OpenGL and
Direct3D, one of the Microsoft DirectX multimedia programming interfaces.
OpenGL, an open standard, was originally proposed and defined by Silicon
Graphics Incorporated. The ongoing development and extension of the OpenGL
standard [Segal and Akeley, 2006], [Kessenich, 2006] is managed by Khronos,
an industry consortium. Direct3D [Blythe, 2006], a de facto standard, is defined
and evolved forward by Microsoft and partners. OpenGL and Direct3D are
similarly structured, and continue to evolve rapidly with GPU hardware advances.
They define a logical graphics processing pipeline that is mapped onto the GPU
hardware and processors, along with programming models and languages for the
programmable pipeline stages.
OpenGL An openstandard graphics API.
Direct3D A graphics API
defined by Microsoft and
partners.
Logical Graphics Pipeline
Figure A.3.1 illustrates the Direct3D 10 logical graphics pipeline. OpenGL has a
similar graphics pipeline structure. The API and logical pipeline provide a streaming
dataflow infrastructure and plumbing for the programmable shader stages, shown in
blue. The 3D application sends the GPU a sequence of vertices grouped into geometric
primitives—points, lines, triangles, and polygons. The input assembler collects
vertices and primitives. The vertex shader program executes per-vertex processing,
Input
Assembler
Vertex
Shader
Sampler
Vertex
Buffer
Index Buffer
Texture
Constant
Geometry
Shader
Sampler
Texture
Constant
Setup &
Rasterizer
Stream
Out
Pixel
Shader
Raster Operations/
Output Merger
Sampler
GPU
Stream
Buffer
Texture
Memory
Constant
Depth
Z-Buffer
Render
Target
Stencil
FIGURE A.3.1 Direct3D 10 graphics pipeline. Each logical pipeline stage maps to GPU hardware or to a GPU processor. Programmable
shader stages are blue, fixed-function blocks are white, and memory objects are grey. Each stage processes a vertex, geometric primitive, or pixel
in a streaming dataflow fashion.
A-14
texture A 1D, 2D, or
3D array that supports
sampled and filtered
lookups with interpolated
coordinates.
Appendix A Graphics and Computing GPUs
including transforming the vertex 3D position into a screen position and lighting the
vertex to determine its color. The geometry shader program executes per-primitive
processing and can add or drop primitives. The setup and rasterizer unit generates
pixel fragments (fragments are potential contributions to pixels) that are covered by
a geometric primitive. The pixel shader program performs per-fragment processing,
including interpolating per-fragment parameters, texturing, and coloring. Pixel
shaders make extensive use of sampled and filtered lookups into large 1D, 2D, or
3D arrays called textures, using interpolated floating-point coordinates. Shaders use
texture accesses for maps, functions, decals, images, and data. The raster operations
processing (or output merger) stage performs Z-buffer depth testing and stencil
testing, which may discard a hidden pixel fragment or replace the pixel’s depth with
the fragment’s depth, and performs a color blending operation that combines the
fragment color with the pixel color and writes the pixel with the blended color.
The graphics API and graphics pipeline provide input, output, memory objects,
and infrastructure for the shader programs that process each vertex, primitive, and
pixel fragment.
Graphics Shader Programs
shader A program that
operates on graphics data
such as a vertex or a pixel
fragment.
shading language
A graphics rendering
language, usually having
a dataflow or streaming
programming model.
Real-time graphics applications use many different shader programs to model
how light interacts with different materials and to render complex lighting and
shadows. Shading languages are based on a dataflow or streaming programming
model that corresponds with the logical graphics pipeline. Vertex shader programs
map the position of triangle vertices onto the screen, altering their position, color,
or orientation. Typically a vertex shader thread inputs a floating-point (x, y, z, w)
vertex position and computes a floating-point (x, y, z) screen position. Geometry
shader programs operate on geometric primitives (such as lines and triangles)
defined by multiple vertices, changing them or generating additional primitives.
Pixel fragment shaders each “shade” one pixel, computing a floating-point red,
green, blue, alpha (RGBA) color contribution to the rendered image at its pixel
sample (x, y) image position. Shaders (and GPUs) use floating-point arithmetic
for all pixel color calculations to eliminate visible artifacts while computing the
extreme range of pixel contribution values encountered while rendering scenes with
complex lighting, shadows, and high dynamic range. For all three types of graphics
shaders, many program instances can be run in parallel, as independent parallel
threads, because each works on independent data, produces independent results,
and has no side effects. Independent vertices, primitives, and pixels further enable
the same graphics program to run on differently sized GPUs that process different
numbers of vertices, primitives, and pixels in parallel. Graphics programs thus scale
transparently to GPUs with different amounts of parallelism and performance.
Users program all three logical graphics threads with a common targeted highlevel language. HLSL (high-level shading language) and Cg (C for graphics) are
commonly used. They have C-like syntax and a rich set of library functions for
matrix operations, trigonometry, interpolation, and texture access and filtering,
but are far from general computing languages: they currently lack general memory
A.3
Programming GPUs
access, pointers, file I/O, and recursion. HLSL and Cg assume that programs live
within a logical graphics pipeline, and thus I/O is implicit. For example, a pixel
fragment shader may expect the geometric normal and multiple texture coordinates
to have been interpolated from vertex values by upstream fixed-function stages and
can simply assign a value to the COLOR output parameter to pass it downstream
to be blended with a pixel at an implied (x, y) position.
The GPU hardware creates a new independent thread to execute a vertex, geometry,
or pixel shader program for every vertex, every primitive, and every pixel fragment. In
video games, the bulk of threads execute pixel shader programs, as there are typically
10 to 20 times or more pixel fragments than vertices, and complex lighting and
shadows require even larger ratios of pixel to vertex shader threads. The graphics
shader programming model drove the GPU architecture to efficiently execute
thousands of independent fine-grained threads on many parallel processor cores.
Pixel Shader Example
Consider the following Cg pixel shader program that implements the “environment
mapping” rendering technique. For each pixel thread, this shader is passed five
parameters, including 2D floating-point texture image coordinates needed to
sample the surface color, and a 3D floating-point vector giving the reflection of
the view direction off the surface. The other three “uniform” parameters do not
vary from one pixel instance (thread) to the next. The shader looks up color in
two texture images: a 2D texture access for the surface color, and a 3D texture
access into a cube map (six images corresponding to the faces of a cube) to obtain
the external world color corresponding to the reflection direction. Then the final
four-component (red, green, blue, alpha) floating-point color is computed using a
weighted average called a “lerp” or linear interpolation function.
void reflection(
float2
float3
out float4
uniform float
uniform sampler2D
uniform samplerCUBE
{
texCoord
reflection_dir
color
shiny,
surfaceMap,
envMap)
: TEXCOORD0,
: TEXCOORD1,
: COLOR,
// Fetch the surface color from a texture
float4 surfaceColor = tex2D(surfaceMap, texCoord);
// Fetch reflected color by sampling a cube map
float4 reflectedColor = texCUBE(environmentMap, reflection_dir);
// Output is weighted average of the two colors
color = lerp(surfaceColor, reflectedColor, shiny);
}
A-15
A-16
Appendix A Graphics and Computing GPUs
Although this shader program is only three lines long, it activates a lot of GPU
hardware. For each texture fetch, the GPU texture subsystem makes multiple
memory accesses to sample image colors in the vicinity of the sampling coordinates,
and then interpolates the final result with floating-point filtering arithmetic. The
multithreaded GPU executes thousands of these lightweight Cg pixel shader threads
in parallel, deeply interleaving them to hide texture fetch and memory latency.
Cg focuses the programmer’s view to a single vertex or primitive or pixel, which
the GPU implements as a single thread; the shader program transparently scales to
exploit thread parallelism on the available processors. Being application-specific, Cg
provides a rich set of useful data types, library functions, and language constructs
to express diverse rendering techniques.
Figure A.3.2 shows skin rendered by a fragment pixel shader. Real skin appears
quite different from flesh-color paint because light bounces around a lot before
re-emerging. In this complex shader, three separate skin layers, each with unique
subsurface scattering behavior, are modeled to give the skin a visual depth and
translucency. Scattering can be modeled by a blurring convolution in a flattened
“texture” space, with red being blurred more than green, and blue blurred less.
FIGURE A.3.2 GPU-rendered image. To give the skin visual depth and translucency, the pixel shader
program models three separate skin layers, each with unique subsurface scattering behavior. It executes 1400
instructions to render the red, green, blue, and alpha color components of each skin pixel fragment.
A.3
Programming GPUs
The compiled Cg shader executes 1400 instructions to compute the color of one
skin pixel.
As GPUs have evolved superior floating-point performance and very high
streaming memory bandwidth for real-time graphics, they have attracted highly
parallel applications beyond traditional graphics. At first, access to this power
was available only by couching an application as a graphics-rendering algorithm,
but this GPGPU approach was often awkward and limiting. More recently,
the CUDA programming model has provided a far easier way to exploit the
scalable high-performance floating-point and memory bandwidth of GPUs with
the C programming language.
Programming Parallel Computing Applications
CUDA, Brook, and CAL are programming interfaces for GPUs that are focused on
data parallel computation rather than on graphics. CAL (Compute Abstraction
Layer) is a low-level assembler language interface for AMD GPUs. Brook is a
streaming language adapted for GPUs by Buck, et. al. [2004]. CUDA, developed by
NVIDIA [2007], is an extension to the C and C++ languages for scalable parallel
programming of manycore GPUs and multicore CPUs. The CUDA programming
model is described below, adapted from an article by Nickolls, Buck, Garland, and
Skadron [2008].
With the new model the GPU excels in data parallel and throughput computing,
executing high performance computing applications as well as graphics applications.
Data Parallel Problem Decomposition
To map large computing problems effectively to a highly parallel processing
architecture, the programmer or compiler decomposes the problem into many
small problems that can be solved in parallel. For example, the programmer partitions a large result data array into blocks and further partitions each block into
elements, such that the result blocks can be computed independently in parallel,
and the elements within each block are computed in parallel. Figure A.3.3 shows
a decomposition of a result data array into a 3 × 2 grid of blocks, where each
block is further decomposed into a 5 × 3 array of elements. The two-level parallel
decomposition maps naturally to the GPU architecture: parallel multiprocessors
compute result blocks, and parallel threads compute result elements.
The programmer writes a program that computes a sequence of result data
grids, partitioning each result grid into coarse-grained result blocks that can be
computed independently in parallel. The program computes each result block with
an array of fine-grained parallel threads, partitioning the work among threads so
that each computes one or more result elements.
Scalable Parallel Programming with CUDA
The CUDA scalable parallel programming model extends the C and C++
languages to exploit large degrees of parallelism for general applications on highly
parallel multiprocessors, particularly GPUs. Early experience with CUDA shows
A-17
A-18
Appendix A Graphics and Computing GPUs
Sequence
Result Data Grid 1
Step 1:
Block
(0, 0)
Block
(1, 0)
Block
(2, 0)
Block
(0, 1)
Block
(1, 1)
Block
(2, 1)
Result Data Grid 2
Step 2:
Block (1, 1)
Elem Elem Elem Elem Elem
(0, 0) (1, 0) (2, 0) (3, 0) (4, 0)
Elem Elem Elem Elem Elem
(0, 1) (1, 1) (2, 1) (3, 1) (4, 1)
Elem Elem Elem Elem Elem
(0, 2) (1, 2) (2, 2) (3, 2) (4, 2)
FIGURE A.3.3
in parallel.
Decomposing result data into a grid of blocks of elements to be computed
that many sophisticated programs can be readily expressed with a few easily
understood abstractions. Since NVIDIA released CUDA in 2007, developers have
rapidly developed scalable parallel programs for a wide range of applications,
including seismic data processing, computational chemistry, linear algebra, sparse
matrix solvers, sorting, searching, physics models, and visual computing. These
applications scale transparently to hundreds of processor cores and thousands of
concurrent threads. NVIDIA GPUs with the Tesla unified graphics and computing
architecture (described in sections A.4 and A.7) run CUDA C programs, and are
widely available in laptops, PCs, workstations, and servers. The CUDA model is
also applicable to other shared memory parallel processing architectures, including
multicore CPUs [Stratton, 2008].
CUDA provides three key abstractions—a hierarchy of thread groups, shared
memories, and barrier synchronization—that provide a clear parallel structure to conventional C code for one thread of the hierarchy. Multiple levels of threads, memory,
and synchronization provide fine-grained data parallelism and thread parallelism,
nested within coarse-grained data parallelism and task parallelism. The abstractions
guide the programmer to partition the problem into coarse subproblems that can
be solved independently in parallel, and then into finer pieces that can be solved in
parallel. The programming model scales transparently to large numbers of processor cores: a compiled CUDA program executes on any number of processors, and
only the runtime system needs to know the physical processor count.
A.3
A-19
Programming GPUs
The CUDA Paradigm
CUDA is a minimal extension of the C and C++ programming languages. The
programmer writes a serial program that calls parallel kernels, which may be simple
functions or full programs. A kernel executes in parallel across a set of parallel
threads. The programmer organizes these threads into a hierarchy of thread blocks
and grids of thread blocks. A thread block is a set of concurrent threads that can
cooperate among themselves through barrier synchronization and through shared
access to a memory space private to the block. A grid is a set of thread blocks that
may each be executed independently and thus may execute in parallel.
When invoking a kernel, the programmer specifies the number of threads per
block and the number of blocks comprising the grid. Each thread is given a unique
thread ID number threadIdx within its thread block, numbered 0, 1, 2, ...,
blockDim–1, and each thread block is given a unique block ID number blockIdx
within its grid. CUDA supports thread blocks containing up to 512 threads. For
convenience, thread blocks and grids may have 1, 2, or 3 dimensions, accessed via
.x, .y, and .z index fields.
As a very simple example of parallel programming, suppose that we are given
two vectors x and y of n floating-point numbers each and that we wish to compute
the result of y = ax + y for some scalar value a. This is the so-called SAXPY kernel
defined by the BLAS linear algebra library. Figure A.3.4 shows C code for performing this computation on both a serial processor and in parallel using CUDA.
The __global__ declaration specifier indicates that the procedure is a kernel
entry point. CUDA programs launch parallel kernels with the extended function
call syntax:
kernel<<<dimGrid, dimBlock>>>(... parameter list ...);
where dimGrid and dimBlock are three-element vectors of type dim3 that specify
the dimensions of the grid in blocks and the dimensions of the blocks in threads,
respectively. Unspecified dimensions default to one.
In Figure A.3.4, we launch a grid of n threads that assigns one thread to each
element of the vectors and puts 256 threads in each block. Each individual thread
computes an element index from its thread and block IDs and then performs the
desired calculation on the corresponding vector elements. Comparing the serial and
parallel versions of this code, we see that they are strikingly similar. This represents
a fairly common pattern. The serial code consists of a loop where each iteration is
independent of all the others. Such loops can be mechanically transformed into
parallel kernels: each loop iteration becomes an independent thread. By assigning
a single thread to each output element, we avoid the need for any synchronization
among threads when writing results to memory.
The text of a CUDA kernel is simply a C function for one sequential thread.
Thus, it is generally straightforward to write and is typically simpler than writing
parallel code for vector operations. Parallelism is determined clearly and explicitly
by specifying the dimensions of a grid and its thread blocks when launching a
kernel.
kernel A program or
function for one thread,
designed to be executed
by many threads.
thread block A set
of concurrent threads
that execute the same
thread program and may
cooperate to compute a
result.
grid A set of thread
blocks that execute the
same kernel program.
A-20
Appendix A Graphics and Computing GPUs
Computing y = ax + y with a serial loop:
void saxpy_serial(int n, float alpha, float *x, float *y)
{
for(int i = 0; i<n; ++i)
y[i] = alpha*x[i] + y[i];
}
// Invoke serial SAXPY kernel
saxpy_serial(n, 2.0, x, y);
Computing y = ax + y in parallel using CUDA:
__global__
void saxpy_parallel(int n, float alpha, float *x, float *y)
{
int i = blockIdx.x*blockDim.x + threadIdx.x;
if( i<n )
y[i] = alpha*x[i] + y[i];
}
// Invoke parallel SAXPY kernel (256 threads per block)
int nblocks = (n + 255) / 256;
saxpy_parallel<<<nblocks, 256>>>(n, 2.0, x, y);
FIGURE A.3.4 Sequential code (top) in C versus parallel code (bottom) in CUDA for SAXPY
(see Chapter 7). CUDA parallel threads replace the C serial loop—each thread computes the same result as
one loop iteration. The parallel code computes n results with n threads organized in blocks of 256 threads.
synchronization barrier
Threads wait at a synchronization barrier until
all threads in the thread
block arrive at the barrier.
Parallel execution and thread management is automatic. All thread creation,
scheduling, and termination is handled for the programmer by the underlying system. Indeed, a Tesla architecture GPU performs all thread management directly in
hardware. The threads of a block execute concurrently and may synchronize at a
synchronization barrier by calling the __syncthreads() intrinsic. This guarantees that no thread in the block can proceed until all threads in the block have
reached the barrier. After passing the barrier, these threads are also guaranteed to
see all writes to memory performed by threads in the block before the barrier.
Thus, threads in a block may communicate with each other by writing and reading
per-block shared memory at a synchronization barrier.
Since threads in a block may share memory and synchronize via barriers, they
will reside together on the same physical processor or multiprocessor. The number
of thread blocks can, however, greatly exceed the number of processors. The CUDA
thread programming model virtualizes the processors and gives the programmer the
flexibility to parallelize at whatever granularity is most convenient. Virtualization
A.3
Programming GPUs
into threads and thread blocks allows intuitive problem decompositions, as the
number of blocks can be dictated by the size of the data being processed rather
than by the number of processors in the system. It also allows the same CUDA
program to scale to widely varying numbers of processor cores.
To manage this processing element virtualization and provide scalability, CUDA
requires that thread blocks be able to execute independently. It must be possible to
execute blocks in any order, in parallel or in series. Different blocks have no means of
direct communication, although they may coordinate their activities using atomic
memory operations on the global memory visible to all threads—by atomically
incrementing queue pointers, for example. This independence requirement allows
thread blocks to be scheduled in any order across any number of cores, making
the CUDA model scalable across an arbitrary number of cores as well as across a
variety of parallel architectures. It also helps to avoid the possibility of deadlock.
An application may execute multiple grids either independently or dependently.
Independent grids may execute concurrently, given sufficient hardware resources.
Dependent grids execute sequentially, with an implicit interkernel barrier between
them, thus guaranteeing that all blocks of the first grid complete before any block
of the second, dependent grid begins.
Threads may access data from multiple memory spaces during their execution.
Each thread has a private local memory. CUDA uses local memory for threadprivate variables that do not fit in the thread’s registers, as well as for stack frames
and register spilling. Each thread block has a shared memory, visible to all threads
of the block, which has the same lifetime as the block. Finally, all threads have
access to the same global memory. Programs declare variables in shared and
global memory with the __shared__ and __device__ type qualifiers. On a
Tesla architecture GPU, these memory spaces correspond to physically separate
memories: per-block shared memory is a low-latency on-chip RAM, while global
memory resides in the fast DRAM on the graphics board.
Shared memory is expected to be a low-latency memory near each processor,
much like an L1 cache. It can therefore provide high-performance communication
and data sharing among the threads of a thread block. Since it has the same lifetime
as its corresponding thread block, kernel code will typically initialize data in
shared variables, compute using shared variables, and copy shared memory results
to global memory. Thread blocks of sequentially dependent grids communicate
via global memory, using it to read input and write results.
Figure A.3.5 diagrams the nested levels of threads, thread blocks, and grids of
thread blocks. It further shows the corresponding levels of memory sharing: local,
shared, and global memories for per-thread, per-thread-block, and per-application
data sharing.
A program manages the global memory space visible to kernels through calls
to the CUDA runtime, such as cudaMalloc() and cudaFree(). Kernels may
execute on a physically separate device, as is the case when running kernels on
the GPU. Consequently, the application must use cudaMemcpy() to copy data
between the allocated space and the host system memory.
A-21
atomic memory
operation A memory
read, modify, write
operation sequence that
completes without any
intervening access.
local memory Per-thread
local memory private to
the thread.
shared memory Perblock memory shared by
all threads of the block.
global memory Perapplication memory
shared by all threads.
A-22
Appendix A Graphics and Computing GPUs
Thread
per-Thread Local Memory
Thread Block
per-Block
Shared Memory
Grid 0
Sequence
...
Inter-Grid Synchronization
Global Memory
Grid 1
...
FIGURE A.3.5 Nested granularity levels—thread, thread block, and grid—have corresponding
memory sharing levels—local, shared, and global. Per-thread local memory is private to the thread.
Per-block shared memory is shared by all threads of the block. Per-application global memory is shared by all
threads.
single-program multiple
data (SPMD) A style
of parallel programming
model in which all
threads execute the same
program. SPMD threads
typically coordinate with
barrier synchronization.
The CUDA programming model is similar in style to the familiar single-program
multiple data (SPMD) model—it expresses parallelism explicitly, and each kernel
executes on a fixed number of threads. However, CUDA is more flexible than most
realizations of SPMD, because each kernel call dynamically creates a new grid with
the right number of thread blocks and threads for that application step. The programmer can use a convenient degree of parallelism for each kernel, rather than
having to design all phases of the computation to use the same number of threads.
Figure A.3.6 shows an example of an SPMD-like CUDA code sequence. It first
instantiates kernelF on a 2D grid of 3 × 2 blocks where each 2D thread block consists of 5 × 3 threads. It then instantiates kernelG on a 1D grid of four 1D thread
blocks with six threads each. Because kernelG depends on the results of kernelF,
they are separated by an interkernel synchronization barrier.
The concurrent threads of a thread block express fine-grained data parallelism and thread parallelism. The independent thread blocks of a grid express
A.3
Programming GPUs
kernelF 2D Grid is 3 2 thread blocks; each block is 5 3 threads
Sequence
kernelF<<<(3, 2), (5, 3)>>>(params);
Block 0, 0
Block 1, 0
Block 2, 0
Block 0, 1
Block 1, 1
Block 2, 1
Block 1, 1
Thread 0, 0
Thread 1, 0
Thread 2, 0
Thread 3, 0
Thread 4, 0
Thread 0, 1
Thread 1, 1
Thread 2, 1
Thread 3, 1
Thread 4, 1
Thread 0, 2
Thread 1, 2
Thread 2, 2
Thread 3, 2
Thread 4, 2
Interkernel Synchronization Barrier
kernelG 1D Grid is 4 thread blocks, each block is 6 threads
Block 0
Block 1
Block 2
Block 3
kernelG<<<4, 6>>>(params);
Block 2
Thread 0
Thread 1
Thread 2
Thread 3
Thread 4
Thread 5
FIGURE A.3.6 Sequence of kernel F instantiated on a 2D grid of 2D thread blocks, an interkernel
synchronization barrier, followed by kernel G on a 1D grid of 1D thread blocks.
coarse-grained data parallelism. Independent grids express coarse-grained task
parallelism. A kernel is simply C code for one thread of the hierarchy.
Restrictions
For efficiency, and to simplify its implementation, the CUDA programming model
has some restrictions. Threads and thread blocks may only be created by invoking
a parallel kernel, not from within a parallel kernel. Together with the required
independence of thread blocks, this makes it possible to execute CUDA programs
A-23
A-24
Appendix A Graphics and Computing GPUs
with a simple scheduler that introduces minimal runtime overhead. In fact, the
Tesla GPU architecture implements hardware management and scheduling of
threads and thread blocks.
Task parallelism can be expressed at the thread block level but is difficult to
express within a thread block because thread synchronization barriers operate on
all the threads of the block. To enable CUDA programs to run on any number of
processors, dependencies among thread blocks within the same kernel grid are not
allowed—blocks must execute independently. Since CUDA requires that thread
blocks be independent and allows blocks to be executed in any order, combining
results generated by multiple blocks must in general be done by launching a second
kernel on a new grid of thread blocks (although thread blocks may coordinate their
activities using atomic memory operations on the global memory visible to all
threads—by atomically incrementing queue pointers, for example).
Recursive function calls are not currently allowed in CUDA kernels. Recursion
is unattractive in a massively parallel kernel, because providing stack space for the
tens of thousands of threads that may be active would require substantial amounts
of memory. Serial algorithms that are normally expressed using recursion, such as
quicksort, are typically best implemented using nested data parallelism rather than
explicit recursion.
To support a heterogeneous system architecture combining a CPU and a
GPU, each with its own memory system, CUDA programs must copy data and
results between host memory and device memory. The overhead of CPU–GPU
interaction and data transfers is minimized by using DMA block transfer engines
and fast interconnects. Compute-intensive problems large enough to need a GPU
performance boost amortize the overhead better than small problems.
Implications for Architecture
The parallel programming models for graphics and computing have driven
GPU architecture to be different than CPU architecture. The key aspects of GPU
programs driving GPU processor architecture are:
■
Extensive use of fine-grained data parallelism: Shader programs describe how
to process a single pixel or vertex, and CUDA programs describe how to
compute an individual result.
■
Highly threaded programming model: A shader thread program processes a
single pixel or vertex, and a CUDA thread program may generate a single
result. A GPU must create and execute millions of such thread programs per
frame, at 60 frames per second.
■
Scalability: A program must automatically increase its performance when
provided with additional processors, without recompiling.
■
Intensive floating-point (or integer) computation.
■
Support of high throughput computations.
A.4 Multithreaded Multiprocessor Architecture
A.4
Multithreaded Multiprocessor
Architecture
To address different market segments, GPUs implement scalable numbers of
multiprocessors—in fact, GPUs are multiprocessors composed of multiprocessors.
Furthermore, each multiprocessor is highly multithreaded to execute many finegrained vertex and pixel shader threads efficiently. A quality basic GPU has two to
four multiprocessors, while a gaming enthusiast’s GPU or computing platform has
dozens of them. This section looks at the architecture of one such multithreaded
multiprocessor, a simplified version of the NVIDIA Tesla streaming multiprocessor
(SM) described in Section A.7.
Why use a multiprocessor, rather than several independent processors? The
parallelism within each multiprocessor provides localized high performance and
supports extensive multithreading for the fine-grained parallel programming
models described in Section A.3. The individual threads of a thread block execute
together within a multiprocessor to share data. The multithreaded multiprocessor
design we describe here has eight scalar processor cores in a tightly coupled architecture, and executes up to 512 threads (the SM described in Section A.7 executes
up to 768 threads). For area and power efficiency, the multiprocessor shares large
complex units among the eight processor cores, including the instruction cache,
the multithreaded instruction unit, and the shared memory RAM.
Massive Multithreading
GPU processors are highly multithreaded to achieve several goals:
■
Cover the latency of memory loads and texture fetches from DRAM
■
Support fine-grained parallel graphics shader programming models
■
Support fine-grained parallel computing programming models
■
Virtualize the physical processors as threads and thread blocks to provide
transparent scalability
■
Simplify the parallel programming model to writing a serial program for one
thread
Memory and texture fetch latency can require hundreds of processor clocks,
because GPUs typically have small streaming caches rather than large working-set
caches like CPUs. A fetch request generally requires a full DRAM access latency
plus interconnect and buffering latency. Multithreading helps cover the latency
with useful computing—while one thread is waiting for a load or texture fetch
to complete, the processor can execute another thread. The fine-grained parallel
programming models provide literally thousands of independent threads that can
keep many processors busy despite the long memory latency seen by individual
threads.
A-25
A-26
Appendix A Graphics and Computing GPUs
A graphics vertex or pixel shader program is a program for a single thread that
processes a vertex or a pixel. Similarly, a CUDA program is a C program for a
single thread that computes a result. Graphics and computing programs instantiate
many parallel threads to render complex images and compute large result arrays.
To dynamically balance shifting vertex and pixel shader thread workloads, each
multiprocessor concurrently executes multiple different thread programs and
different types of shader programs.
To support the independent vertex, primitive, and pixel programming model of
graphics shading languages and the single-thread programming model of CUDA
C/C++, each GPU thread has its own private registers, private per-thread memory,
program counter, and thread execution state, and can execute an independent code
path. To efficiently execute hundreds of concurrent lightweight threads, the GPU
multiprocessor is hardware multithreaded—it manages and executes hundreds
of concurrent threads in hardware without scheduling overhead. Concurrent
threads within thread blocks can synchronize at a barrier with a single instruction.
Lightweight thread creation, zero-overhead thread scheduling, and fast barrier
synchronization efficiently support very fine-grained parallelism.
Multiprocessor Architecture
A unified graphics and computing multiprocessor executes vertex, geometry, and
pixel fragment shader programs, and parallel computing programs. As Figure A.4.1
shows, the example multiprocessor consists of eight scalar processor (SP) cores
each with a large multithreaded register file (RF), two special function units (SFU),
a multithreaded instruction unit, an instruction cache, a read-only constant cache,
and a shared memory.
The 16 KB shared memory holds graphics data buffers and shared computing
data. CUDA variables declared as __shared__ reside in the shared memory. To
map the logical graphics pipeline workload through the multiprocessor multiple
times, as shown in Section A.2, vertex, geometry, and pixel threads have independent
input and output buffers, and workloads arrive and depart independently of thread
execution.
Each SP core contains scalar integer and floating-point arithmetic units that
execute most instructions. The SP is hardware multithreaded, supporting up to
64 threads. Each pipelined SP core executes one scalar instruction per thread per
clock, which ranges from 1.2 GHz to 1.6 GHz in different GPU products. Each
SP core has a large register file (RF) of 1024 general-purpose 32-bit registers,
partitioned among its assigned threads. Programs declare their register demand,
typically 16 to 64 scalar 32-bit registers per thread. The SP can concurrently run
many threads that use a few registers or fewer threads that use more registers. The
compiler optimizes register allocation to balance the cost of spilling registers versus
the cost of fewer threads. Pixel shader programs often use 16 or fewer registers,
enabling each SP to run up to 64 pixel shader threads to cover long-latency texture
fetches. Compiled CUDA programs often need 32 registers per thread, limiting
each SP to 32 threads, which limits such a kernel program to 256 threads per thread
block on this example multiprocessor, rather than its maximum of 512 threads.
A.4 Multithreaded Multiprocessor Architecture
A-27
Multithreaded Multiprocessor
Instruction Cache
Multiprocessor
Controller
Multithreaded Instruction Unit
Constant Cache
SFU
Work Interface
SP
SP
SP
SP
SP
SP
SP
SP
RF
RF
RF
RF
RF
RF
RF
RF
SFU
Input
Interface
Interconnection Network
Output
Interface
Shared Memory
Texture
Interface
Memory
Interface
FIGURE A.4.1 Multithreaded multiprocessor with eight scalar processor (SP) cores. The
eight SP cores each have a large multithreaded register file (RF) and share an instruction cache, multithreaded
instruction issue unit, constant cache, two special function units (SFUs), interconnection network, and a
multibank shared memory.
The pipelined SFUs execute thread instructions that compute special functions
and interpolate pixel attributes from primitive vertex attributes. These instructions
can execute concurrently with instructions on the SPs. The SFU is described later.
The multiprocessor executes texture fetch instructions on the texture unit via the
texture interface, and uses the memory interface for external memory load, store,
and atomic access instructions. These instructions can execute concurrently with
instructions on the SPs. Shared memory access uses a low-latency interconnection
network between the SP processors and the shared memory banks.
Single-Instruction Multiple-Thread (SIMT)
To manage and execute hundreds of threads running several different programs
efficiently, the multiprocessor employs a single-instruction multiple-thread
(SIMT) architecture. It creates, manages, schedules, and executes concurrent
threads in groups of parallel threads called warps. The term warp originates from
weaving, the first parallel thread technology. The photograph in Figure A.4.2 shows
a warp of parallel threads emerging from a loom. This example multiprocessor
uses a SIMT warp size of 32 threads, executing four threads in each of the eight
single-instruction
multiple-thread (SIMT)
A processor architecture
that applies one
instruction to multiple
independent threads in
parallel.
warp The set of parallel
threads that execute the
same instruction together
in a SIMT architecture.
Appendix A Graphics and Computing GPUs
Photo: Judy Schoonmaker
A-28
SIMT multithreaded
instruction scheduler
time
warp 8 instruction 11
warp 1 instruction 42
warp 3 instruction 95
warp 8 instruction 12
warp 3 instruction 96
warp 1 instruction 43
FIGURE A.4.2 SIMT multithreaded warp scheduling. The scheduler selects a ready warp and issues
an instruction synchronously to the parallel threads composing the warp. Because warps are independent,
the scheduler may select a different warp each time.
SP cores over four clocks. The Tesla SM multiprocessor described in Section A.7
also uses a warp size of 32 parallel threads, executing four threads per SP core for
efficiency on plentiful pixel threads and computing threads. Thread blocks consist
of one or more warps.
This example SIMT multiprocessor manages a pool of 16 warps, a total of 512
threads. Individual parallel threads composing a warp are the same type and start
together at the same program address, but are otherwise free to branch and execute
independently. At each instruction issue time, the SIMT multithreaded instruction
unit selects a warp that is ready to execute its next instruction, then issues that
instruction to the active threads of that warp. A SIMT instruction is broadcast
synchronously to the active parallel threads of a warp; individual threads may be
inactive due to independent branching or predication. In this multiprocessor, each
SP scalar processor core executes an instruction for four individual threads of a
warp using four clocks, reflecting the 4:1 ratio of warp threads to cores.
SIMT processor architecture is akin to single-instruction multiple data (SIMD)
design, which applies one instruction to multiple data lanes, but differs in that
SIMT applies one instruction to multiple independent threads in parallel, not just
A.4 Multithreaded Multiprocessor Architecture
to multiple data lanes. An instruction for a SIMD processor controls a vector of
multiple data lanes together, whereas an instruction for a SIMT processor controls
an individual thread, and the SIMT instruction unit issues an instruction to a warp
of independent parallel threads for efficiency. The SIMT processor finds data-level
parallelism among threads at runtime, analogous to the way a superscalar processor
finds instruction-level parallelism among instructions at runtime.
A SIMT processor realizes full efficiency and performance when all threads
of a warp take the same execution path. If threads of a warp diverge via a datadependent conditional branch, execution serializes for each branch path taken, and
when all paths complete, the threads converge to the same execution path. For equal
length paths, a divergent if-else code block is 50% efficient. The multiprocessor uses
a branch synchronization stack to manage independent threads that diverge and
converge. Different warps execute independently at full speed regardless of whether
they are executing common or disjoint code paths. As a result, SIMT GPUs are
dramatically more efficient and flexible on branching code than earlier GPUs, as
their warps are much narrower than the SIMD width of prior GPUs.
In contrast with SIMD vector architectures, SIMT enables programmers to write
thread-level parallel code for individual independent threads, as well as data-parallel
code for many coordinated threads. For program correctness, the programmer can
essentially ignore the SIMT execution attributes of warps; however, substantial
performance improvements can be realized by taking care that the code seldom
requires threads in a warp to diverge. In practice, this is analogous to the role of
cache lines in traditional codes: cache line size can be safely ignored when designing
for correctness but must be considered in the code structure when designing for
peak performance.
SIMT Warp Execution and Divergence
The SIMT approach of scheduling independent warps is more flexible than the
scheduling of previous GPU architectures. A warp comprises parallel threads
of the same type: vertex, geometry, pixel, or compute. The basic unit of pixel
fragment shader processing is the 2-by-2 pixel quad implemented as four pixel
shader threads. The multiprocessor controller packs the pixel quads into a warp. It
similarly groups vertices and primitives into warps, and packs computing threads
into a warp. A thread block comprises one or more warps. The SIMT design shares
the instruction fetch and issue unit efficiently across parallel threads of a warp, but
requires a full warp of active threads to get full performance efficiency.
This unified multiprocessor schedules and executes multiple warp types
concurrently, allowing it to concurrently execute vertex and pixel warps. Its warp
scheduler operates at less than the processor clock rate, because there are four thread
lanes per processor core. During each scheduling cycle, it selects a warp to execute
a SIMT warp instruction, as shown in Figure A.4.2. An issued warp-instruction
executes as four sets of eight threads over four processor cycles of throughput. The
processor pipeline uses several clocks of latency to complete each instruction. If the
number of active warps times the clocks per warp exceeds the pipeline latency, the
A-29
A-30
Appendix A Graphics and Computing GPUs
programmer can ignore the pipeline latency. For this multiprocessor, a round-robin
schedule of eight warps has a period of 32 cycles between successive instructions
for the same warp. If the program can keep 256 threads active per multiprocessor,
instruction latencies up to 32 cycles can be hidden from an individual sequential
thread. However, with few active warps, the processor pipeline depth becomes
visible and may cause processors to stall.
A challenging design problem is implementing zero-overhead warp scheduling
for a dynamic mix of different warp programs and program types. The instruction
scheduler must select a warp every four clocks to issue one instruction per clock
per thread, equivalent to an IPC of 1.0 per processor core. Because warps are
independent, the only dependencies are among sequential instructions from the
same warp. The scheduler uses a register dependency scoreboard to qualify warps
whose active threads are ready to execute an instruction. It prioritizes all such ready
warps and selects the highest priority one for issue. Prioritization must consider
warp type, instruction type, and the desire to be fair to all active warps.
Managing Threads and Thread Blocks
cooperative thread
array (CTA) A set
of concurrent threads
that executes the same
thread program and may
cooperate to compute
a result. A GPU CTA
implements a CUDA
thread block.
The multiprocessor controller and instruction unit manage threads and thread
blocks. The controller accepts work requests and input data and arbitrates access
to shared resources, including the texture unit, memory access path, and I/O paths.
For graphics workloads, it creates and manages three types of graphics threads
concurrently: vertex, geometry, and pixel. Each of the graphics work types have
independent input and output paths. It accumulates and packs each of these input
work types into SIMT warps of parallel threads executing the same thread program.
It allocates a free warp, allocates registers for the warp threads, and starts warp
execution in the multiprocessor. Every program declares its per-thread register
demand; the controller starts a warp only when it can allocate the requested register
count for the warp threads. When all the threads of the warp exit, the controller
unpacks the results and frees the warp registers and resources.
The controller creates cooperative thread arrays (CTAs) which implement
CUDA thread blocks as one or more warps of parallel threads. It creates a CTA
when it can create all CTA warps and allocate all CTA resources. In addition to
threads and registers, a CTA requires allocating shared memory and barriers. The
program declares the required capacities, and the controller waits until it can
allocate those amounts before launching the CTA. Then it creates CTA warps at the
warp scheduling rate, so that a CTA program starts executing immediately at full
multiprocessor performance. The controller monitors when all threads of a CTA
have exited, and frees the CTA shared resources and its warp resources.
Thread Instructions
The SP thread processors execute scalar instructions for individual threads, unlike
earlier GPU vector instruction architectures, which executed four-component
vector instructions for each vertex or pixel shader program. Vertex programs
A.4 Multithreaded Multiprocessor Architecture
generally compute (x, y, z, w) position vectors, while pixel shader programs
compute (red, green, blue, alpha) color vectors. However, shader programs are
becoming longer and more scalar, and it is increasingly difficult to fully occupy
even two components of a legacy GPU four-component vector architecture. In
effect, the SIMT architecture parallelizes across 32 independent pixel threads,
rather than parallelizing the four vector components within a pixel. CUDA C/C++
programs have predominantly scalar code per thread. Previous GPUs employed
vector packing (e.g., combining subvectors of work to gain efficiency) but that
complicated the scheduling hardware as well as the compiler. Scalar instructions
are simpler and compiler friendly. Texture instructions remain vector based, taking
a source coordinate vector and returning a filtered color vector.
To support multiple GPUs with different binary microinstruction formats,
high-level graphics and computing language compilers generate intermediate
assembler-level instructions (e.g., Direct3D vector instructions or PTX scalar
instructions), which are then optimized and translated to binary GPU microinstructions. The NVIDIA PTX (parallel thread execution) instruction set definition
[2007] provides a stable target ISA for compilers, and provides compatibility over
several generations of GPUs with evolving binary microinstruction-set architectures. The optimizer readily expands Direct3D vector instructions to multiple scalar binary microinstructions. PTX scalar instructions translate nearly one to one
with scalar binary microinstructions, although some PTX instructions expand to
multiple binary microinstructions, and multiple PTX instructions may fold into
one binary microinstruction. Because the intermediate assembler-level instructions use virtual registers, the optimizer analyzes data dependencies and allocates
real registers. The optimizer eliminates dead code, folds instructions together when
feasible, and optimizes SIMT branch diverge and converge points.
Instruction Set Architecture (ISA)
The thread ISA described here is a simplified version of the Tesla architecture PTX
ISA, a register-based scalar instruction set comprising floating-point, integer, logical,
conversion, special functions, flow control, memory access, and texture operations.
Figure A.4.3 lists the basic PTX GPU thread instructions; see the NVIDIA PTX
specification [2007] for details. The instruction format is:
opcode.type d, a, b, c;
where d is the destination operand, a, b, c are source operands, and .type is one of:
Type
.type Specifier
Untyped bits 8, 16, 32, and 64 bits
.b8, .b16, .b32, .b64
Unsigned integer 8, 16, 32, and 64 bits
.u8, .u16, .u32, .u64
Signed integer 8, 16, 32, and 64 bits
.s8, .s16, .s32, .s64
Floating-point 16, 32, and 64 bits
.f16, .f32, .f64
A-31
A-32
Appendix A Graphics and Computing GPUs
Basic PTX GPU Thread Instructions
Group
Arithmetic
Instruction
Example
arithmetic .type = .s32, .u32, .f32, .s64, .u64, .f64
add.type
add.f32 d, a, b
d =
sub.type
sub.f32 d, a, b
d =
mul.type
mul.f32 d, a, b
d =
mad.type
mad.f32 d, a, b, c
d =
div.type
div.f32 d, a, b
d =
rem.type
rem.u32 d, a, b
d =
abs.type
abs.f32 d, a
d =
neg.type
neg.f32 d, a
d =
min.type
min.f32 d, a, b
d =
max.type
max.f32 d, a, b
d =
setp.cmp.type
setp.lt.f32 p, a, b
p =
numeric .cmp = eq, ne, lt, le, gt, ge; unordered cmp = equ,
Meaning
Comments
a + b;
a – b;
a * b;
multiply-add
a * b + c;
multiple microinstructions
a / b;
integer remainder
a % b;
|a|;
0 - a;
floating selects non-NaN
(a < b)? a:b;
floating selects non-NaN
(a > b)? a:b;
compare and set predicate
(a < b);
neu, ltu, leu, gtu, geu, num, nan
move
mov.b32 d, a
d = a;
select with predicate
selp.f32 d, a, b, p
d = p? a: b;
convert atype to dtype
cvt.f32.s32 d, a
d = convert(a);
special .type = .f32 (some .f64)
rcp.type
reciprocal
rcp.f32 d, a
d = 1/a;
sqrt.type
square root
sqrt.f32 d, a
d = sqrt(a);
rsqrt.type
reciprocal square root
rsqrt.f32 d, a
d = 1/sqrt(a);
sin.type
sine
sin.f32 d, a
d = sin(a);
cos.type
cosine
cos.f32 d, a
d = cos(a);
lg2.type
binary logarithm
lg2.f32 d, a
d = log(a)/log(2)
ex2.type
binary exponential
ex2.f32 d, a
d = 2 ** a;
logic.type = .pred,.b32, .b64
and.type
and.b32 d, a, b
d = a & b;
or.type
or.b32 d, a, b
d = a | b;
xor.type
xor.b32 d, a, b
d = a ^ b;
not.type
one’s complement
not.b32 d, a, b
d = ~a;
cnot.type
C logical not
cnot.b32 d, a, b
d = (a==0)? 1:0;
shl.type
shift left
shl.b32 d, a, b
d = a << b;
shr.type
shift right
shr.s32 d, a, b
d = a >> b;
memory .space = .global, .shared, .local, .const; .type = .b8, .u8, .s8, .b16, .b32, .b64
ld.space.type
load from memory space
ld.global.b32 d, [a+off]
d = *(a+off);
st.space.type
store to memory space
st.shared.b32 [d+off], a
*(d+off) = a;
tex.nd.dtyp.btype
texture lookup
tex.2d.v4.f32.f32 d, a, b
d = tex2d(a, b);
atom.spc.op.type
atomic read-modify-write
atom.global.add.u32 d,[a], b
atomic { d = *a;
atom.global.cas.b32 d,[a], b, c
*a = op(*a, b); } operation
mov.type
selp.type
cvt.dtype.atype
Special
Function
Logical
Memory
Access
Control
Flow
atom .op = and, or, xor, add, min, max, exch, cas; .spc = .global; .type = .b32
branch
conditional branch
@p bra target
if (p) goto
target;
call
call (ret), func, (params)
ret = func(params); call function
ret
return from function call
ret
return;
bar.sync
barrier synchronization
bar.sync d
wait for threads
exit
terminate thread execution
exit
exit;
FIGURE A.4.3
Basic PTX GPU thread instructions.
A.4 Multithreaded Multiprocessor Architecture
Source operands are scalar 32-bit or 64-bit values in registers, an immediate
value, or a constant; predicate operands are 1-bit Boolean values. Destinations are
registers, except for store to memory. Instructions are predicated by prefixing them
with @p or @!p, where p is a predicate register. Memory and texture instructions
transfer scalars or vectors of two to four components, up to 128 bits total. PTX
instructions specify the behavior of one thread.
The PTX arithmetic instructions operate on 32-bit and 64-bit floating-point,
signed integer, and unsigned integer types. Recent GPUs support 64-bit double
precision floating-point; see Section A.6. On current GPUs, PTX 64-bit integer
and logical instructions are translated to two or more binary microinstructions
that perform 32-bit operations. The GPU special function instructions are limited
to 32-bit floating-point. The thread control flow instructions are conditional
branch, function call and return, thread exit, and bar.sync (barrier
synchronization). The conditional branch instruction @p bra target uses a
predicate register p (or !p) previously set by a compare and set predicate setp
instruction to determine whether the thread takes the branch or not. Other
instructions can also be predicated on a predicate register being true or false.
Memory Access Instructions
The tex instruction fetches and filters texture samples from 1D, 2D, and 3D
texture arrays in memory via the texture subsystem. Texture fetches generally use
interpolated floating-point coordinates to address a texture. Once a graphics pixel
shader thread computes its pixel fragment color, the raster operations processor
blends it with the pixel color at its assigned (x, y) pixel position and writes the final
color to memory.
To support computing and C/C++ language needs, the Tesla PTX ISA implements
memory load/store instructions. They use integer byte addressing with register plus
offset address arithmetic to facilitate conventional compiler code optimizations.
Memory load/store instructions are common in processors, but are a significant
new capability in the Tesla architecture GPUs, as prior GPUs provided only the
texture and pixel accesses required by the graphics APIs.
For computing, the load/store instructions access three read/write memory
spaces that implement the corresponding CUDA memory spaces in Section A.3:
■
Local memory for per-thread private addressable temporary data (implemented in external DRAM)
■
Shared memory for low-latency access to data shared by cooperating threads
in the same CTA/thread block (implemented in on-chip SRAM)
■
Global memory for large data sets shared by all threads of a computing
application (implemented in external DRAM)
The memory load/store instructions ld.global, st.global, ld.shared,
st.shared, ld.local, and st.local access the global, shared, and local memory spaces. Computing programs use the fast barrier synchronization instruction
bar.sync to synchronize threads within a CTA/thread block that communicate
with each other via shared and global memory.
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Appendix A Graphics and Computing GPUs
To improve memory bandwidth and reduce overhead, the local and global load/
store instructions coalesce individual parallel thread requests from the same SIMT
warp together into a single memory block request when the addresses fall in the
same block and meet alignment criteria. Coalescing memory requests provides a
significant performance boost over separate requests from individual threads. The
multiprocessor’s large thread count, together with support for many outstanding
load requests, helps cover load-to-use latency for local and global memory implemented in external DRAM.
The latest Tesla architecture GPUs also provide efficient atomic memory operations on memory with the atom.op.u32 instructions, including integer operations
add, min, max, and, or, xor, exchange, and cas (compare-and-swap) operations, facilitating parallel reductions and parallel data structure management.
Barrier Synchronization for Thread Communication
Fast barrier synchronization permits CUDA programs to communicate frequently
via shared memory and global memory by simply calling __syncthreads(); as
part of each interthread communication step. The synchronization intrinsic function generates a single bar.sync instruction. However, implementing fast barrier
synchronization among up to 512 threads per CUDA thread block is a challenge.
Grouping threads into SIMT warps of 32 threads reduces the synchronization
difficulty by a factor of 32. Threads wait at a barrier in the SIMT thread scheduler
so they do not consume any processor cycles while waiting. When a thread executes
a bar.sync instruction, it increments the barrier’s thread arrival counter and the
scheduler marks the thread as waiting at the barrier. Once all the CTA threads
arrive, the barrier counter matches the expected terminal count, and the scheduler
releases all the threads waiting at the barrier and resumes executing threads.
Streaming Processor (SP)
The multithreaded streaming processor (SP) core is the primary thread instruction
processor in the multiprocessor. Its register file (RF) provides 1024 scalar
32-bit registers for up to 64 threads. It executes all the fundamental floating-point
operations, including add.f32, mul.f32, mad.f32 (floating multiply-add),
min.f32, max.f32, and setp.f32 (floating compare and set predicate). The
floating-point add and multiply operations are compatible with the IEEE 754
standard for single precision FP numbers, including not-a-number (NaN) and
infinity values. The SP core also implements all of the 32-bit and 64-bit integer
arithmetic, comparison, conversion, and logical PTX instructions in Figure A.4.3.
The floating-point add and mul operations employ IEEE round-to-nearest-even
as the default rounding mode. The mad.f32 floating-point multiply-add operation
performs a multiplication with truncation, followed by an addition with roundto-nearest-even. The SP flushes input denormal operands to sign-preserved-zero.
Results that underflow the target output exponent range are flushed to signpreserved-zero after rounding.
A.4 Multithreaded Multiprocessor Architecture
Special Function Unit (SFU)
Certain thread instructions can execute on the SFUs, concurrently with other
thread instructions executing on the SPs. The SFU implements the special function
instructions of Figure A.4.3, which compute 32-bit floating-point approximations
to reciprocal, reciprocal square root, and key transcendental functions. It also
implements 32-bit floating-point planar attribute interpolation for pixel shaders,
providing accurate interpolation of attributes such as color, depth, and texture
coordinates.
Each pipelined SFU generates one 32-bit floating-point special function result
per cycle; the two SFUs per multiprocessor execute special function instructions
at a quarter the simple instruction rate of the eight SPs. The SFUs also execute the
mul.f32 multiply instruction concurrently with the eight SPs, increasing the peak
computation rate up to 50% for threads with a suitable instruction mixture.
For functional evaluation, the Tesla architecture SFU employs quadratic
interpolation based on enhanced minimax approximations for approximating the
reciprocal, reciprocal square-root, log2x, 2x, and sin/cos functions. The accuracy of
the function estimates ranges from 22 to 24 mantissa bits. See Section A.6 for more
details on SFU arithmetic.
Comparing with Other Multiprocessors
Compared with SIMD vector architectures such as x86 SSE, the SIMT multiprocessor can execute individual threads independently, rather than always executing
them together in synchronous groups. SIMT hardware finds data parallelism among
independent threads, whereas SIMD hardware requires the software to express data
parallelism explicitly in each vector instruction. A SIMT machine executes a warp
of 32 threads synchronously when the threads take the same execution path, yet
can execute each thread independently when they diverge. The advantage is significant because SIMT programs and instructions simply describe the behavior of a
single independent thread, rather than a SIMD data vector of four or more data
lanes. Yet the SIMT multiprocessor has SIMD-like efficiency, spreading the area
and cost of one instruction unit across the 32 threads of a warp and across the eight
streaming processor cores. SIMT provides the performance of SIMD together with
the productivity of multithreading, avoiding the need to explicitly code SIMD vectors for edge conditions and partial divergence.
The SIMT multiprocessor imposes little overhead because it is hardware
multithreaded with hardware barrier synchronization. That allows graphics
shaders and CUDA threads to express very fine-grained parallelism. Graphics and
CUDA programs use threads to express fine-grained data parallelism in a perthread program, rather than forcing the programmer to express it as SIMD vector
instructions. It is simpler and more productive to develop scalar single-thread code
than vector code, and the SIMT multiprocessor executes the code with SIMD-like
efficiency.
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Appendix A Graphics and Computing GPUs
Coupling eight streaming processor cores together closely into a multiprocessor
and then implementing a scalable number of such multiprocessors makes a twolevel multiprocessor composed of multiprocessors. The CUDA programming model
exploits the two-level hierarchy by providing individual threads for fine-grained
parallel computations, and by providing grids of thread blocks for coarse-grained
parallel operations. The same thread program can provide both fine-grained and
coarse-grained operations. In contrast, CPUs with SIMD vector instructions must
use two different programming models to provide fine-grained and coarse-grained
operations: coarse-grained parallel threads on different cores, and SIMD vector
instructions for fine-grained data parallelism.
Multithreaded Multiprocessor Conclusion
The example GPU multiprocessor based on the Tesla architecture is highly
multithreaded, executing a total of up to 512 lightweight threads concurrently to
support fine-grained pixel shaders and CUDA threads. It uses a variation on SIMD
architecture and multithreading called SIMT (single-instruction multiple-thread)
to efficiently broadcast one instruction to a warp of 32 parallel threads, while
permitting each thread to branch and execute independently. Each thread executes
its instruction stream on one of the eight streaming processor (SP) cores, which are
multithreaded up to 64 threads.
The PTX ISA is a register-based load/store scalar ISA that describes the execution
of a single thread. Because PTX instructions are optimized and translated to binary
microinstructions for a specific GPU, the hardware instructions can evolve rapidly
without disrupting compilers and software tools that generate PTX instructions.
A.5
Parallel Memory System
Outside of the GPU itself, the memory subsystem is the most important determiner
of the performance of a graphics system. Graphics workloads demand very high
transfer rates to and from memory. Pixel write and blend (read-modify-write)
operations, depth buffer reads and writes, and texture map reads, as well as
command and object vertex and attribute data reads, comprise the majority of
memory traffic.
Modern GPUs are highly parallel, as shown in Figure A.2.5. For example, the
GeForce 8800 can process 32 pixels per clock, at 600 MHz. Each pixel typically
requires a color read and write and a depth read and write of a 4-byte pixel. Usually
an average of two or three texels of four bytes each are read to generate the pixel’s
color. So for a typical case, there is a demand of 28 bytes times 32 pixels = 896 bytes
per clock. Clearly the bandwidth demand on the memory system is enormous.
A.5 Parallel Memory System
To supply these requirements, GPU memory systems have the following
characteristics:
■
They are wide, meaning there are a large number of pins to convey data
between the GPU and its memory devices, and the memory array itself
comprises many DRAM chips to provide the full total data bus width.
■
They are fast, meaning aggressive signaling techniques are used to maximize
the data rate (bits/second) per pin.
■
GPUs seek to use every available cycle to transfer data to or from the memory
array. To achieve this, GPUs specifically do not aim to minimize latency
to the memory system. High throughput (utilization efficiency) and short
latency are fundamentally in conflict.
■
Compression techniques are used, both lossy, of which the programmer
must be aware, and lossless, which is invisible to the application and
opportunistic.
■
Caches and work coalescing structures are used to reduce the amount of
off-chip traffic needed and to ensure that cycles spent moving data are used
as fully as possible.
DRAM Considerations
GPUs must take into account the unique characteristics of DRAM. DRAM chips
are internally arranged as multiple (typically four to eight) banks, where each bank
includes a power-of-2 number of rows (typically around 16,384), and each row
contains a power-of-2 number of bits (typically 8192). DRAMs impose a variety of
timing requirements on their controlling processor. For example, dozens of cycles
are required to activate one row, but once activated, the bits within that row are
randomly accessible with a new column address every four clocks. Double-data
rate (DDR) synchronous DRAMs transfer data on both rising and falling edges
of the interface clock (see Chapter 5). So a 1 GHz clocked DDR DRAM transfers
data at 2 gigabits per second per data pin. Graphics DDR DRAMs usually have
32 bidirectional data pins, so eight bytes can be read or written from the DRAM
per clock.
GPUs internally have a large number of generators of memory traffic.
Different stages of the logical graphics pipeline each have their own request
streams: command and vertex attribute fetch, shader texture fetch and load/
store, and pixel depth and color read-write. At each logical stage, there are often
multiple independent units to deliver the parallel throughput. These are each
independent memory requestors. When viewed at the memory system, there
are an enormous number of uncorrelated requests in flight. This is a natural
mismatch to the reference pattern preferred by the DRAMs. A solution is for
the GPU’s memory controller to maintain separate heaps of traffic bound for
A-37
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Appendix A Graphics and Computing GPUs
different DRAM banks, and wait until enough traffic for a particular DRAM
row is pending before activating that row and transferring all the traffic at once.
Note that accumulating pending requests, while good for DRAM row locality
and thus efficient use of the data bus, leads to longer average latency as seen by
the requestors whose requests spend time waiting for others. The design must
take care that no particular request waits too long, otherwise some processing
units can starve waiting for data and ultimately cause neighboring processors to
become idle.
GPU memory subsystems are arranged as multiple memory partitions, each of
which comprises a fully independent memory controller and one or two DRAM
devices that are fully and exclusively owned by that partition. To achieve the best
load balance and therefore approach the theoretical performance of n partitions,
addresses are finely interleaved evenly across all memory partitions. The partition
interleaving stride is typically a block of a few hundred bytes. The number of
memory partitions is designed to balance the number of processors and other
memory requesters.
Caches
GPU workloads typically have very large working sets—on the order of hundreds
of megabytes to generate a single graphics frame. Unlike with CPUs, it is not
practical to construct caches on chips large enough to hold anything close to the
full working set of a graphics application. Whereas CPUs can assume very high
cache hit rates (99.9% or more), GPUs experience hit rates closer to 90% and must
therefore cope with many misses in flight. While a CPU can reasonably be designed
to halt while waiting for a rare cache miss, a GPU needs to proceed with misses and
hits intermingled. We call this a streaming cache architecture.
GPU caches must deliver very high-bandwidth to their clients. Consider the case
of a texture cache. A typical texture unit may evaluate two bilinear interpolations
for each of four pixels per clock cycle, and a GPU may have many such texture
units all operating independently. Each bilinear interpolation requires four
separate texels, and each texel might be a 64-bit value. Four 16-bit components
are typical. Thus, total bandwidth is 2 × 4 × 4 × 64 = 2048 bits per clock. Each
separate 64-bit texel is independently addressed, so the cache needs to handle
32 unique addresses per clock. This naturally favors a multibank and/or multiport
arrangement of SRAM arrays.
MMU
Modern GPUs are capable of translating virtual addresses to physical addresses.
On the GeForce 8800, all processing units generate memory addresses in a
40-bit virtual address space. For computing, load and store thread instructions use
32-bit byte addresses, which are extended to a 40-bit virtual address by adding a
40-bit offset. A memory management unit performs virtual to physical address
A.5 Parallel Memory System
translation; hardware reads the page tables from local memory to respond to misses
on behalf of a hierarchy of translation lookaside buffers spread out among the
processors and rendering engines. In addition to physical page bits, GPU page table
entries specify the compression algorithm for each page. Page sizes range from 4 to
128 kilobytes.
Memory Spaces
As introduced in Section A.3, CUDA exposes different memory spaces to allow the
programmer to store data values in the most performance-optimal way. For the
following discussion, NVIDIA Tesla architecture GPUs are assumed.
Global memory
Global memory is stored in external DRAM; it is not local to any one physical
streaming multiprocessor (SM) because it is meant for communication among
different CTAs (thread blocks) in different grids. In fact, the many CTAs that
reference a location in global memory may not be executing in the GPU at the
same time; by design, in CUDA a programmer does not know the relative order
in which CTAs are executed. Because the address space is evenly distributed
among all memory partitions, there must be a read/write path from any streaming
multiprocessor to any DRAM partition.
Access to global memory by different threads (and different processors) is not
guaranteed to have sequential consistency. Thread programs see a relaxed memory
ordering model. Within a thread, the order of memory reads and writes to the same
address is preserved, but the order of accesses to different addresses may not be
preserved. Memory reads and writes requested by different threads are unordered.
Within a CTA, the barrier synchronization instruction bar.sync can be used
to obtain strict memory ordering among the threads of the CTA. The membar
thread instruction provides a memory barrier/fence operation that commits prior
memory accesses and makes them visible to other threads before proceeding.
Threads can also use the atomic memory operations described in Section A.4 to
coordinate work on memory they share.
Shared memory
Per-CTA shared memory is only visible to the threads that belong to that CTA,
and shared memory only occupies storage from the time a CTA is created to the
time it terminates. Shared memory can therefore reside on-chip. This approach has
many benefits. First, shared memory traffic does not need to compete with limited
off-chip bandwidth needed for global memory references. Second, it is practical to
build very high-bandwidth memory structures on-chip to support the read/write
demands of each streaming multiprocessor. In fact, the shared memory is closely
coupled to the streaming multiprocessor.
A-39
A-40
Appendix A Graphics and Computing GPUs
Each streaming multiprocessor contains eight physical thread processors.
During one shared memory clock cycle, each thread processor can process two
threads’ worth of instructions, so 16 threads’ worth of shared memory requests
must be handled in each clock. Because each thread can generate its own addresses,
and the addresses are typically unique, the shared memory is built using 16
independently addressable SRAM banks. For common access patterns, 16 banks are
sufficient to maintain throughput, but pathological cases are possible; for example,
all 16 threads might happen to access a different address on one SRAM bank. It
must be possible to route a request from any thread lane to any bank of SRAM, so
a 16-by-16 interconnection network is required.
Local Memory
Per-thread local memory is private memory visible only to a single thread. Local
memory is architecturally larger than the thread’s register file, and a program
can compute addresses into local memory. To support large allocations of local
memory (recall the total allocation is the per-thread allocation times the number
of active threads), local memory is allocated in external DRAM.
Although global and per-thread local memory reside off-chip, they are wellsuited to being cached on-chip.
Constant Memory
Constant memory is read-only to a program running on the SM (it can be written
via commands to the GPU). It is stored in external DRAM and cached in the SM.
Because commonly most or all threads in a SIMT warp read from the same address
in constant memory, a single address lookup per clock is sufficient. The constant
cache is designed to broadcast scalar values to threads in each warp.
Texture Memory
Texture memory holds large read-only arrays of data. Textures for computing have
the same attributes and capabilities as textures used with 3D graphics. Although
textures are commonly two-dimensional images (2D arrays of pixel values), 1D
(linear) and 3D (volume) textures are also available.
A compute program references a texture using a tex instruction. Operands
include an identifier to name the texture, and 1, 2, or 3 coordinates based on the
texture dimensionality. The floating-point coordinates include a fractional portion
that specifies a sample location often in between texel locations. Noninteger
coordinates invoke a bilinear weighted interpolation of the four closest values (for
a 2D texture) before the result is returned to the program.
Texture fetches are cached in a streaming cache hierarchy designed to optimize
throughput of texture fetches from thousands of concurrent threads. Some programs
use texture fetches as a way to cache global memory.
A.6
Floating-point
Surfaces
Surface is a generic term for a one-dimensional, two-dimensional, or threedimensional array of pixel values and an associated format. A variety of formats
are defined; for example, a pixel may be defined as four 8-bit RGBA integer
components, or four 16-bit floating-point components. A program kernel does
not need to know the surface type. A tex instruction recasts its result values as
floating-point, depending on the surface format.
Load/Store Access
Load/store instructions with integer byte addressing enable the writing and compiling of programs in conventional languages like C and C++. CUDA programs use
load/store instructions to access memory.
To improve memory bandwidth and reduce overhead, the local and global load/
store instructions coalesce individual parallel thread requests from the same warp
together into a single memory block request when the addresses fall in the same
block and meet alignment criteria. Coalescing individual small memory requests
into large block requests provides a significant performance boost over separate
requests. The large thread count, together with support for many outstanding load
requests, helps cover load-to-use latency for local and global memory implemented
in external DRAM.
ROP
As shown in Figure A.2.5, NVIDIA Tesla architecture GPUs comprise a scalable
streaming processor array (SPA), which performs all of the GPU’s programmable
calculations, and a scalable memory system, which comprises external DRAM
control and fixed function Raster Operation Processors (ROPs) that perform color
and depth framebuffer operations directly on memory. Each ROP unit is paired
with a specific memory partition. ROP partitions are fed from the SMs via an
interconnection network. Each ROP is responsible for depth and stencil tests and
updates, as well as color blending. The ROP and memory controllers cooperate
to implement lossless color and depth compression (up to 8:1) to reduce external
bandwidth demand. ROP units also perform atomic operations on memory.
A.6
Floating-point Arithmetic
GPUs today perform most arithmetic operations in the programmable processor
cores using IEEE 754–compatible single precision 32-bit floating-point operations
(see Chapter 3). The fixed-point arithmetic of early GPUs was succeeded by 16-bit,
24-bit, and 32-bit floating-point, then IEEE 754–compatible 32-bit floating-point.
A-41
A-42
Appendix A Graphics and Computing GPUs
Some fixed-function logic within a GPU, such as texture-filtering hardware,
continues to use proprietary numeric formats. Recent GPUs also provide IEEE 754
compatible double precision 64-bit floating-point instructions.
Supported Formats
half precision A 16-bit
binary floating-point
format, with 1 sign bit,
5-bit exponent, 10-bit
fraction, and an implied
integer bit.
The IEEE 754 standard for floating-point arithmetic [2008] specifies basic and
storage formats. GPUs use two of the basic formats for computation, 32-bit and
64-bit binary floating-point, commonly called single precision and double precision. The standard also specifies a 16-bit binary storage floating-point format,
half precision. GPUs and the Cg shading language employ the narrow 16-bit
half data format for efficient data storage and movement, while maintaining high
dynamic range. GPUs perform many texture filtering and pixel blending computations at half precision within the texture filtering unit and the raster operations
unit. The OpenEXR high dynamic-range image file format developed by Industrial
Light and Magic [2003] uses the identical half format for color component values
in computer imaging and motion picture applications.
Basic Arithmetic
multiply-add (MAD)
A single floating-point
instruction that performs
a compound operation:
multiplication followed
by addition.
Common single precision floating-point operations in GPU programmable cores
include addition, multiplication, multiply-add, minimum, maximum, compare,
set predicate, and conversions between integer and floating-point numbers.
Floating-point instructions often provide source operand modifiers for negation
and absolute value.
The floating-point addition and multiplication operations of most GPUs today
are compatible with the IEEE 754 standard for single precision FP numbers, including not-a-number (NaN) and infinity values. The FP addition and multiplication operations use IEEE round-to-nearest-even as the default rounding mode.
To increase floating-point instruction throughput, GPUs often use a compound
multiply-add instruction (mad). The multiply-add operation performs FP multiplication with truncation, followed by FP addition with round-to-nearest-even. It
provides two floating-point operations in one issuing cycle, without requiring the
instruction scheduler to dispatch two separate instructions, but the computation
is not fused and truncates the product before the addition. This makes it different
from the fused multiply-add instruction discussed in Chapter 3 and later in this
section. GPUs typically flush denormalized source operands to sign-preserved zero,
and they flush results that underflow the target output exponent range to signpreserved zero after rounding.
Specialized Arithmetic
GPUs provide hardware to accelerate special function computation, attribute
interpolation, and texture filtering. Special function instructions include cosine,
A.6
Floating-point
sine, binary exponential, binary logarithm, reciprocal, and reciprocal square root.
Attribute interpolation instructions provide efficient generation of pixel attributes,
derived from plane equation evaluation. The special function unit (SFU)
introduced in Section A.4 computes special functions and interpolates planar
attributes [Oberman and Siu, 2005].
Several methods exist for evaluating special functions in hardware. It has been
shown that quadratic interpolation based on Enhanced Minimax Approximations
is a very efficient method for approximating functions in hardware, including
reciprocal, reciprocal square-root, log2x, 2x, sin, and cos.
We can summarize the method of SFU quadratic interpolation. For a binary
input operand X with n-bit significand, the significand is divided into two parts:
Xu is the upper part containing m bits, and Xl is the lower part containing n-m bits.
The upper m bits Xu are used to consult a set of three lookup tables to return three
finite-word coefficients C0, C1, and C2. Each function to be approximated requires
a unique set of tables. These coefficients are used to approximate a given function
f(X) in the range Xu <= X < Xu + 2−m by evaluating the expression:
f(X) = C0 + C1 Xl + C2 Xl2
The accuracy of each of the function estimates ranges from 22 to 24 significand
bits. Example function statistics are shown in Figure A.6.1.
The IEEE 754 standard specifies exact-rounding requirements for division
and square root, however, for many GPU applications, exact compliance is not
required. Rather, for those applications, higher computational throughput is more
important than last-bit accuracy. For the SFU special functions, the CUDA math
library provides both a full accuracy function and a fast function with the SFU
instruction accuracy.
Another specialized arithmetic operation in a GPU is attribute interpolation.
Key attributes are usually specified for vertices of primitives that make up a scene
to be rendered. Example attributes are color, depth, and texture coordinates. These
attributes must be interpolated in the (x,y) screen space as needed to determine the
Input
interval
Accuracy
(good bits)
ULP*
error
% exactly
rounded
Monotonic
1/x
[1, 2)
24.02
0.98
87
Yes
1/sqrt(x)
[1, 4)
23.40
1.52
78
Yes
2
[0, 1)
22.51
1.41
74
Yes
log2x
[1, 2)
22.57
N/A**
N/A
Yes
[0, /2)
22.47
N/A
N/A
No
Function
x
sin/cos
*
ULP: unit in the last place. **N/A: not applicable.
FIGURE A.6.1 Special function approximation statistics. For the NVIDIA GeForce 8800 special
function unit (SFU).
A-43
special function unit
(SFU) A hardware unit
that computes special
functions and interpolates
planar attributes.
A-44
Appendix A Graphics and Computing GPUs
values of the attributes at each pixel location. The value of a given attribute U in an
(x,y) plane can be expressed using plane equations of the form:
U(x, y) = Aux + Buy + Cu
where A, B, and C are interpolation parameters associated with each attribute U.
The interpolation parameters A, B, and C are all represented as single precision
floating-point numbers.
Given the need for both a function evaluator and an attribute interpolator in a
pixel shader processor, a single SFU that performs both functions for efficiency can
be designed. Both functions use a sum of products operation to interpolate results,
and the number of terms to be summed in both functions is very similar.
Texture Operations
Texture mapping and filtering is another key set of specialized floating-point
arithmetic operations in a GPU. The operations used for texture mapping include:
1. Receive texture address (s, t) for the current screen pixel (x, y), where s and t
are single precision floating-point numbers.
MIP-map A Latin phrase
multum in parvo, or
much in a small space.
A MIP-map contains
precalculated images of
different resolutions, used
to increase rendering
speed and reduce artifacts.
2. Compute the level of detail to identify the correct texture MIP-map level.
3. Compute the trilinear interpolation fraction.
4. Scale texture address (s, t) for the selected MIP-map level.
5. Access memory and retrieve desired texels (texture elements).
6. Perform filtering operation on texels.
Texture mapping requires a significant amount of floating-point computation
for full-speed operation, much of which is done at 16-bit half precision. As an
example, the GeForce 8800 Ultra delivers about 500 GFLOPS of proprietary format
floating-point computation for texture mapping instructions, in addition to its
conventional IEEE single precision floating-point instructions. For more details on
texture mapping and filtering, see Foley and van Dam [1995].
Performance
The floating-point addition and multiplication arithmetic hardware is fully
pipelined, and latency is optimized to balance delay and area. While pipelined, the
throughput of the special functions is less than the floating-point addition and
multiplication operations. Quarter-speed throughput for the special functions
is typical performance in modern GPUs, with one SFU shared by four SP cores.
In contrast, CPUs typically have significantly lower throughput for similar
functions, such as division and square root, albeit with more accurate results. The
attribute interpolation hardware is typically fully pipelined to enable full-speed
pixel shaders.
A.6
Floating-point
Double precision
Newer GPUs such as the Tesla T10P also support IEEE 754 64-bit double precision
operations in hardware. Standard floating-point arithmetic operations in double
precision include addition, multiplication, and conversions between different
floating-point and integer formats. The 2008 IEEE 754 floating-point standard
includes specification for the fused-multiply-add operation (FMA), as discussed in
Chapter 3. The FMA operation performs a floating-point multiplication followed
by an addition, with a single rounding. The fused multiplication and addition
operations retain full accuracy in intermediate calculations. This behavior enables
more accurate floating-point computations involving the accumulation of products, including dot products, matrix multiplication, and polynomial evaluation.
The FMA instruction also enables efficient software implementations of exactly
rounded division and square root, removing the need for a hardware division or
square root unit.
A double precision hardware FMA unit implements 64-bit addition, multiplication, conversions, and the FMA operation itself. The architecture of a double
64
64
A
64
B
53
C
53
53
Inversion
Multiplier Array
53 x 53
Sum
Alignment
shifter
Shifted
C
Carry
Exp
Diff
161
3-2 CSA 161 bits
Sum
Carry
Carry Propagate Adder
Complementer
Normalizer
Rounder
FIGURE A.6.2 Double precision fused-multiply-add (FMA) unit. Hardware to implement floatingpoint A × B + C for double precision.
A-45
A-46
Appendix A Graphics and Computing GPUs
precision FMA unit enables full-speed denormalized number support on both
inputs and outputs. Figure A.6.2 shows a block diagram of an FMA unit.
As shown in Figure A.6.2, the significands of A and B are multiplied to form
a 106-bit product, with the results left in carry-save form. In parallel, the 53-bit
addend C is conditionally inverted and aligned to the 106-bit product. The sum and
carry results of the 106-bit product are summed with the aligned addend through a
161-bit-wide carry-save adder (CSA). The carry-save output is then summed together
in a carry-propagate adder to produce an unrounded result in nonredundant, two’s
complement form. The result is conditionally recomplemented, so as to return a
result in sign-magnitude form. The complemented result is normalized, and then
it is rounded to fit within the target format.
A.7
Real Stuff: The NVIDIA GeForce 8800
The NVIDIA GeForce 8800 GPU, introduced in November 2006, is a unified vertex
and pixel processor design that also supports parallel computing applications
written in C using the CUDA parallel programming model. It is the first implementation of the Tesla unified graphics and computing architecture described in
Section A.4 and in Lindholm, Nickolls, Oberman, and Montrym [2008]. A family
of Tesla architecture GPUs addresses the different needs of laptops, desktops, workstations, and servers.
Streaming Processor Array (SPA)
The GeForce 8800 GPU shown in Figure A.7.1 contains 128 streaming processor
(SP) cores organized as 16 streaming multiprocessors (SMs). Two SMs share
a texture unit in each texture/processor cluster (TPC). An array of eight TPCs
makes up the streaming processor array (SPA), which executes all graphics shader
programs and computing programs.
The host interface unit communicates with the host CPU via the PCI-Express
bus, checks command consistency, and performs context switching. The input
assembler collects geometric primitives (points, lines, triangles). The work distribution blocks dispatch vertices, pixels, and compute thread arrays to the TPCs in
the SPA. The TPCs execute vertex and geometry shader programs and computing
programs. Output geometric data is sent to the viewport/clip/setup/raster/zcull
block to be rasterized into pixel fragments that are then redistributed back into the
SPA to execute pixel shader programs. Shaded pixels are sent across the interconnection network for processing by the ROP units. The network also routes texture memory read requests from the SPA to DRAM and reads data from DRAM
through a level-2 cache back to the SPA.
A.7
Host CPU
Bridge
A-47
Real Stuff: The NVIDIA GeForce 8800
System Memory
GPU
Host Interface
Viewport/Clip/
Setup/Raster/
ZCull
Input Assembler
TPC
SM
TPC
SM
SM
TPC
SM
Compute Work
Distribution
Pixel Work
Distribution
Vertex Work
Distribution
SPA
High-Definition
Video Processors
SM
TPC
SM
SM
SM
TPC
SM
TPC
SM
SM
TPC
SM
SM
TPC
SM
SM
SM
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
SP SP SP SP
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
Shared
Memory
Texture Unit
Tex L1
Interconnection Network
ROP
L2
DRAM
ROP
L2
DRAM
ROP
L2
DRAM
ROP
L2
DRAM
ROP
L2
DRAM
ROP
L2
DRAM
Interface
Display
FIGURE A.7.1 NVIDIA Tesla unified graphics and computing GPU architecture. This GeForce 8800 has 128 streaming processor
(SP) cores in 16 streaming multiprocessors (SM), arranged in eight texture/processor clusters (TPC). The processors connect with six 64-bit-wide
DRAM partitions via an interconnection network. Other GPUs implementing the Tesla architecture vary the number of SP cores, SMs, DRAM
partitions, and other units.
Texture/Processor Cluster (TPC)
Each TPC contains a geometry controller, an SM controller (SMC), two streaming
multiprocessors (SMs), and a texture unit as shown in Figure A.7. 2.
The geometry controller maps the logical graphics vertex pipeline into recirculation on the physical SMs by directing all primitive and vertex attribute and
topology flow in the TPC.
The SMC controls multiple SMs, arbitrating the shared texture unit, load/store
path, and I/O path. The SMC serves three graphics workloads simultaneously:
vertex, geometry, and pixel.
The texture unit processes a texture instruction for one vertex, geometry, or pixel
quad, or four compute threads per cycle. Texture instruction sources are texture
coordinates, and the outputs are weighted samples, typically a four-component
(RGBA) floating-point color. The texture unit is deeply pipelined. Although it
A-48
Appendix A Graphics and Computing GPUs
TPC
SM
Geometry Controller
I-Cache
SMC
MT Issue
SM
SM
I-Cache
I-Cache
MT Issue
MT Issue
C-Cache
C-Cache
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SP
SFU SFU
SFU SFU
Shared
Memory
Shared
Memory
Texture Unit
Tex L1
C-Cache
SP
SP
SP
SP
SP
SP
SP
SP
SFU
SFU
Shared
Memory
FIGURE A.7.2 Texture/processor cluster (TPC) and a streaming multiprocessor (SM). Each SM has eight streaming processor
(SP) cores, two SFUs, and a shared memory.
contains a streaming cache to capture filtering locality, it streams hits mixed with
misses without stalling.
Streaming Multiprocessor (SM)
The SM is a unified graphics and computing multiprocessor that executes vertex,
geometry, and pixel-fragment shader programs and parallel computing programs.
The SM consists of eight SP thread processor cores, two SFUs, a multithreaded
instruction fetch and issue unit (MT issue), an instruction cache, a read-only constant
cache, and a 16 KB read/write shared memory. It executes scalar instructions for
individual threads.
The GeForce 8800 Ultra clocks the SP cores and SFUs at 1.5 GHz, for a peak of
36 GFLOPS per SM. To optimize power and area efficiency, some SM nondatapath
units operate at half the SP clock rate.
A.7
Real Stuff: The NVIDIA GeForce 8800
To efficiently execute hundreds of parallel threads while running several
different programs, the SM is hardware multithreaded. It manages and executes up
to 768 concurrent threads in hardware with zero scheduling overhead. Each thread
has its own thread execution state and can execute an independent code path.
A warp consists of up to 32 threads of the same type—vertex, geometry, pixel,
or compute. The SIMT design, previously described in Section A.4, shares the SM
instruction fetch and issue unit efficiently across 32 threads but requires a full warp
of active threads for full performance efficiency.
The SM schedules and executes multiple warp types concurrently. Each issue
cycle, the scheduler selects one of the 24 warps to execute a SIMT warp instruction.
An issued warp instruction executes as four sets of 8 threads over four processor
cycles. The SP and SFU units execute instructions independently, and by issuing
instructions between them on alternate cycles, the scheduler can keep both fully
occupied. A scoreboard qualifies each warp for issue each cycle. The instruction
scheduler prioritizes all ready warps and selects the one with highest priority for
issue. Prioritization considers warp type, instruction type, and “fairness” to all
warps executing in the SM.
The SM executes cooperative thread arrays (CTAs) as multiple concurrent
warps which access a shared memory region allocated dynamically for the CTA.
Instruction Set
Threads execute scalar instructions, unlike previous GPU vector instruction
architectures. Scalar instructions are simpler and compiler friendly. Texture
instructions remain vector based, taking a source coordinate vector and returning
a filtered color vector.
The register-based instruction set includes all the floating-point and integer
arithmetic, transcendental, logical, flow control, memory load/store, and texture
instructions listed in the PTX instruction table of Figure A.4.3. Memory load/store
instructions use integer byte addressing with register-plus-offset address arithmetic.
For computing, the load/store instructions access three read-write memory spaces:
local memory for per-thread, private, temporary data; shared memory for lowlatency per-CTA data shared by the threads of the CTA; and global memory for
data shared by all threads. Computing programs use the fast barrier synchronization bar.sync instruction to synchronize threads within a CTA that communicate
with each other via shared and global memory. The latest Tesla architecture GPUs
implement PTX atomic memory operations, which facilitate parallel reductions
and parallel data structure management.
Streaming Processor (SP)
The multithreaded SP core is the primary thread processor, as introduced in
Section A.4. Its register file provides 1024 scalar 32-bit registers for up to 96
threads (more threads than the example SP of Section A.4). Its floating-point
A-49
A-50
Appendix A Graphics and Computing GPUs
add and multiply operations are compatible with the IEEE 754 standard for
single precision FP numbers, including not-a-number (NaN) and infinity. The
add and multiply operations use IEEE round-to-nearest-even as the default
rounding mode. The SP core also implements all of the 32-bit and 64-bit integer
arithmetic, comparison, conversion, and logical PTX instructions in Figure A.4.3.
The processor is fully pipelined, and latency is optimized to balance delay
and area.
Special Function Unit (SFU)
The SFU supports computation of both transcendental functions and planar
attribute interpolation. As described in Section A.6, it uses quadratic interpolation based on enhanced minimax approximations to approximate the reciprocal,
reciprocal square root, log2x, 2x, and sin/cos functions at one result per cycle. The
SFU also supports pixel attribute interpolation such as color, depth, and texture
coordinates at four samples per cycle.
Rasterization
Geometry primitives from the SMs go in their original round-robin input order
to the viewport/clip/setup/raster/zcull block. The viewport and clip units clip
the primitives to the view frustum and to any enabled user clip planes, and then
transform the vertices into screen (pixel) space.
Surviving primitives then go to the setup unit, which generates edge equations
for the rasterizer. A coarse-rasterization stage generates all pixel tiles that are at
least partially inside the primitive. The zcull unit maintains a hierarchical z surface,
rejecting pixel tiles if they are conservatively known to be occluded by previously
drawn pixels. The rejection rate is up to 256 pixels per clock. Pixels that survive
zcull then go to a fine-rasterization stage that generates detailed coverage information and depth values.
The depth test and update can be performed ahead of the fragment shader, or
after, depending on current state. The SMC assembles surviving pixels into warps
to be processed by an SM running the current pixel shader. The SMC then sends
surviving pixel and associated data to the ROP.
Raster Operations Processor (ROP) and Memory System
Each ROP is paired with a specific memory partition. For each pixel fragment
emitted by a pixel shader program, ROPs perform depth and stencil testing and
updates, and in parallel, color blending and updates. Lossless color compression (up
to 8:1) and depth compression (up to 8:1) are used to reduce DRAM bandwidth.
Each ROP has a peak rate of four pixels per clock and supports 16-bit floatingpoint and 32-bit floating-point HDR formats. ROPs support double-rate-depth
processing when color writes are disabled.
A.7
Real Stuff: The NVIDIA GeForce 8800
Antialiasing support includes up to 16× multisampling and supersampling. The
coverage-sampling antialiasing (CSAA) algorithm computes and stores Boolean
coverage at up to 16 samples and compresses redundant color, depth, and stencil
information into the memory footprint and a bandwidth of four or eight samples
for improved performance.
The DRAM memory data bus width is 384 pins, arranged in six independent
partitions of 64 pins each. Each partition supports double-data-rate DDR2 and
graphics-oriented GDDR3 protocols at up to 1.0 GHz, yielding a bandwidth of
about 16 GB/s per partition, or 96 GB/s.
The memory controllers support a wide range of DRAM clock rates, protocols,
device densities, and data bus widths. Texture and load/store requests can occur
between any TPC and any memory partition, so an interconnection network routes
requests and responses.
Scalability
The Tesla unified architecture is designed for scalability. Varying the number of
SMs, TPCs, ROPs, caches, and memory partitions provides the right balance for
different performance and cost targets in GPU market segments. Scalable link
interconnect (SLI) connects multiple GPUs, providing further scalability.
Performance
The GeForce 8800 Ultra clocks the SP thread processor cores and SFUs at 1.5 GHz,
for a theoretical operation peak of 576 GFLOPS. The GeForce 8800 GTX has a
1.35 GHz processor clock and a corresponding peak of 518 GFLOPS.
The following three sections compare the performance of a GeForce 8800 GPU
with a multicore CPU on three different applications—dense linear algebra, fast
Fourier transforms, and sorting. The GPU programs and libraries are compiled
CUDA C code. The CPU code uses the single precision multithreaded Intel MKL
10.0 library to leverage SSE instructions and multiple cores.
Dense Linear Algebra Performance
Dense linear algebra computations are fundamental in many applications. Volkov
and Demmel [2008] present GPU and CPU performance results for single precision
dense matrix-matrix multiplication (the SGEMM routine) and LU, QR, and
Cholesky matrix factorizations. Figure A.7.3 compares GFLOPS rates on SGEMM
dense matrix-matrix multiplication for a GeForce 8800 GTX GPU with a quad-core
CPU. Figure A.7.4 compares GFLOPS rates on matrix factorization for a GPU with
a quad-core CPU.
Because SGEMM matrix-matrix multiply and similar BLAS3 routines are the
bulk of the work in matrix factorization, their performance sets an upper bound on
factorization rate. As the matrix order increases beyond 200 to 400, the factorization
A-51
Appendix A Graphics and Computing GPUs
A:N64, B:64N
A:NN, B:NN
210
GeForce 8800 GTX
180
GFLOPS
150
120
90
60
Core2 Quad
30
0
64
128
256
512
1024
2048
4096
8192
N
FIGURE A.7.3 SGEMM dense matrix-matrix multiplication performance rates. The graph
shows single precision GFLOPS rates achieved in multiplying square N×N matrices (solid lines) and thin
N×64 and 64×N matrices (dashed lines). Adapted from Figure 6 of Volkov and Demmel [2008]. The black
lines are a 1.35 GHz GeForce 8800 GTX using Volkov’s SGEMM code (now in NVIDIA CUBLAS 2.0) on
matrices in GPU memory. The blue lines are a quad-core 2.4 GHz Intel Core2 Quad Q6600, 64-bit Linux,
Intel MKL 10.0 on matrices in CPU memory.
Cholesky
LU
QR
210
Du
o
180
Co
re
2
150
00
G
TX
+
120
88
90
eF
or
ce
GFLOPS
60
uad
Core2 Q
G
A-52
30
0
64
128
256
512
1024
2048
4096
8192
16384
Order of Matrix
FIGURE A.7.4 Dense matrix factorization performance rates. The graph shows GFLOPS rates
achieved in matrix factorizations using the GPU and using the CPU alone. Adapted from Figure 7 of Volkov
and Demmel [2008]. The black lines are a 1.35 GHz NVIDIA GeForce 8800 GTX, CUDA 1.1, Windows XP
attached to a 2.67 GHz Intel Core2 Duo E6700 Windows XP, including all CPU–GPU data transfer times. The
blue lines are a quad-core 2.4 GHz Intel Core2 Quad Q6600, 64-bit Linux, Intel MKL 10.0.
A.7
Real Stuff: The NVIDIA GeForce 8800
problem becomes large enough that SGEMM can leverage the GPU parallelism and
overcome the CPU–GPU system and copy overhead. Volkov’s SGEMM matrixmatrix multiply achieves 206 GFLOPS, about 60% of the GeForce 8800 GTX
peak multiply-add rate, while the QR factorization reached 192 GFLOPS, about
4.3 times the quad-core CPU.
FFT Performance
Fast Fourier Transforms are used in many applications. Large transforms and
multidimensional transforms are partitioned into batches of smaller 1D transforms.
Figure A.7.5 compares the in-place 1D complex single precision FFT performance of a 1.35 GHz GeForce 8800 GTX (dating from late 2006) with a 2.8 GHz
quad-Core Intel Xeon E5462 series (code named “Harpertown,” dating from late
2007). CPU performance was measured using the Intel Math Kernel Library (MKL)
10.0 FFT with four threads. GPU performance was measured using the NVIDIA
CUFFT 2.1 library and batched 1D radix-16 decimation-in-frequency FFTs. Both
CPU and GPU throughput performance was measured using batched FFTs, batch
size was 224/n, where n is the transform size. Thus, the workload for every transform size was 128 MB. To determine GFLOPS rate, the number of operations per
transform was taken as 5n log2 n.
GeForce 8800GTX
Xeon 5462
80
70
60
GFLOPS
50
40
30
20
10
0
8
12
6
25
8
2
2
2
8
2
6
6
6
8
4
4
4
4
51 1,02 2,04 4,09 8,19 6,38 2,76 5,53 1,07 2,14 , 28 8,57 7,15 4,30
4
3
6
1
13
26
52 1,04 2,09 4,19
Number of Elements in One Transform
FIGURE A.7.5 Fast Fourier Transform throughput performance. The graph compares the
performance of batched one-dimensional in-place complex FFTs on a 1.35 GHz GeForce 8800 GTX with
a quad-core 2.8 GHz Intel Xeon E5462 series (code named “Harpertown”), 6MB L2 Cache, 4GB Memory,
1600 FSB, Red Hat Linux, Intel MKL 10.0.
A-53
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Appendix A Graphics and Computing GPUs
Sorting Performance
In contrast to the applications just discussed, sort requires far more substantial
coordination among parallel threads, and parallel scaling is correspondingly
harder to obtain. Nevertheless, a variety of well-known sorting algorithms can
be efficiently parallelized to run well on the GPU. Satish, et al. [2008] detail the
design of sorting algorithms in CUDA, and the results they report for radix sort are
summarized below.
Figure A.7.6 compares the parallel sorting performance of a GeForce 8800 Ultra
with an 8-core Intel Clovertown system, both of which date to early 2007. The
CPU cores are distributed between two physical sockets. Each socket contains a
multichip module with twin Core2 chips, and each chip has a 4MB L2 cache. All
sorting routines were designed to sort key-value pairs where both keys and values
are 32-bit integers. The primary algorithm being studied is radix sort, although
the quicksort-based parallel_sort() procedure provided by Intel’s Threading
Building Blocks is also included for comparison. Of the two CPU-based radix sort
codes, one was implemented using only the scalar instruction set and the other
utilizes carefully hand-tuned assembly language routines that take advantage of the
SSE2 SIMD vector instructions.
The graph itself shows the achieved sorting rate—defined as the number of elements sorted divided by the time to sort—for a range of sequence sizes. It is apparent
Sorting Rate (pairs/sec)
Millions
CPU quick sort
GPU radix sort
CPU radix sort (scalar)
CPU radix sort (SIMD)
80
70
60
50
40
30
20
10
0
1,000
10,000
100,000
1,000,000
10,000,000
100,000,000
Sequence Size
FIGURE A.7.6 Parallel sorting performance. This graph compares sorting rates for parallel radix sort implementations on a 1.5 GHz
GeForce 8800 Ultra and an 8-core 2.33 GHz Intel Core2 Xeon E5345 system.
A.8
Real Stuff: Mapping Applications to GPUs
from this graph that the GPU radix sort achieved the highest sorting rate for all
sequences of 8K-elements and larger. In this range, it is on average 2.6 times faster
than the quicksort-based routine and roughly 2 times faster than the radix sort routines, all of which were using the eight available CPU cores. The CPU radix sort performance varies widely, likely due to poor cache locality of its global permutations.
A.8
Real Stuff: Mapping Applications to GPUs
The advent of multicore CPUs and manycore GPUs means that mainstream
processor chips are now parallel systems. Furthermore, their parallelism continues
to scale with Moore’s law. The challenge is to develop mainstream visual computing
and high-performance computing applications that transparently scale their
parallelism to leverage the increasing number of processor cores, much as 3D
graphics applications transparently scale their parallelism to GPUs with widely
varying numbers of cores.
This section presents examples of mapping scalable parallel computing
applications to the GPU using CUDA.
Sparse Matrices
A wide variety of parallel algorithms can be written in CUDA in a fairly
straightforward manner, even when the data structures involved are not simple
regular grids. Sparse matrix-vector multiplication (SpMV) is a good example of
an important numerical building block that can be parallelized quite directly using
the abstractions provided by CUDA. The kernels we discuss below, when combined
with the provided CUBLAS vector routines, make writing iterative solvers such as
the conjugate gradient method straightforward.
A sparse n × n matrix is one in which the number of nonzero entries m is only
a small fraction of the total. Sparse matrix representations seek to store only the
nonzero elements of a matrix. Since it is fairly typical that a sparse n × n matrix will
contain only m = O(n) nonzero elements, this represents a substantial savings in
storage space and processing time.
One of the most common representations for general unstructured sparse
matrices is the compressed sparse row (CSR) representation. The m nonzero
elements of the matrix A are stored in row-major order in an array Av. A second
array Aj records the corresponding column index for each entry of Av. Finally, an
array Ap of n + 1 elements records the extent of each row in the previous arrays; the
entries for row i in Aj and Av extend from index Ap[i] up to, but not including,
index Ap[i + 1]. This implies that Ap[0] will always be 0 and Ap[n] will always
be the number of nonzero elements in the matrix. Figure A.8.1 shows an example
of the CSR representation of a simple matrix.
A-55
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Appendix A Graphics and Computing GPUs
3
0
A = 0
1
0
0
2
0
1
0
4
0
0
0
1
1
a. Sample matrix A
FIGURE A.8.1
Row 0
Row 2
Av[7] = { 3 1 2 4 1
Row 3
1 1 }
Aj[7] = { 0
2
1
2
3
0
Ap[5] = { 0
2
2
5
7
3 }
}
b. CSR representation of matrix
Compressed sparse row (CSR) matrix.
float multiply_row(unsigned int rowsize,
unsigned int *Aj, // column indices for row
float *Av,
// nonzero entries for row
float *x)
// the RHS vector
{
float sum = 0;
for(unsigned int column=0; column<rowsize; ++column)
sum += Av[column] * x[Aj[column]];
return sum;
}
FIGURE A.8.2
Serial C code for a single row of sparse matrix-vector multiply.
Given a matrix A in CSR form and a vector x, we can compute a single row of
the product y = Ax using the multiply_row() procedure shown in Figure A.8.2.
Computing the full product is then simply a matter of looping over all rows and
computing the result for that row using multiply_row(), as in the serial C code
shown in Figure A.8.3.
This algorithm can be translated into a parallel CUDA kernel quite easily. We
simply spread the loop in csrmul_serial() over many parallel threads. Each
thread will compute exactly one row of the output vector y. The code for this kernel
is shown in Figure A.8.4. Note that it looks extremely similar to the serial loop
used in the csrmul_serial() procedure. There are really only two points of
difference. First, the row index for each thread is computed from the block and
thread indices assigned to each thread, eliminating the for-loop. Second, we have a
conditional that only evaluates a row product if the row index is within the bounds
of the matrix (this is necessary since the number of rows n need not be a multiple
of the block size used in launching the kernel).
A.8
Real Stuff: Mapping Applications to GPUs
void csrmul_serial(unsigned int *Ap, unsigned int *Aj,
float *Av, unsigned int num_rows,
float *x, float *y)
{
for(unsigned int row=0; row<num_rows; ++row)
{
unsigned int row_begin = Ap[row];
unsigned int row_end
= Ap[row+1];
y[row] = multiply_row(row_end-row_begin, Aj+row_begin,
Av+row_begin, x);
}
}
FIGURE A.8.3
Serial code for sparse matrix-vector multiply.
__global__
void csrmul_kernel(unsigned int *Ap, unsigned int *Aj,
float *Av, unsigned int num_rows,
float *x, float *y)
{
unsigned int row = blockIdx.x*blockDim.x + threadIdx.x;
if( row<num_rows )
{
unsigned int row_begin = Ap[row];
unsigned int row_end
= Ap[row+1];
y[row] = multiply_row(row_end-row_begin, Aj+row_begin,
Av+row_begin, x);
}
}
FIGURE A.8.4
CUDA version of sparse matrix-vector multiply.
Assuming that the matrix data structures have already been copied to the GPU
device memory, launching this kernel will look like:
unsigned int blocksize = 128; // or any size up to 512
unsigned int nblocks
= (num_rows + blocksize - 1) / blocksize;
csrmul_kernel<<<nblocks,blocksize>>>(Ap, Aj, Av, num_rows, x, y);
A-57
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Appendix A Graphics and Computing GPUs
The pattern that we see here is a very common one. The original serial
algorithm is a loop whose iterations are independent of each other. Such loops
can be parallelized quite easily by simply assigning one or more iterations of the
loop to each parallel thread. The programming model provided by CUDA makes
expressing this type of parallelism particularly straightforward.
This general strategy of decomposing computations into blocks of independent
work, and more specifically breaking up independent loop iterations, is not unique
to CUDA. This is a common approach used in one form or another by various
parallel programming systems, including OpenMP and Intel’s Threading Building
Blocks.
Caching in Shared memory
The SpMV algorithms outlined above are fairly simplistic. There are a number of
optimizations that can be made in both the CPU and GPU codes that can improve
performance, including loop unrolling, matrix reordering, and register blocking.
The parallel kernels can also be reimplemented in terms of data parallel scan
operations presented by Sengupta, et al. [2007].
One of the important architectural features exposed by CUDA is the presence
of the per-block shared memory, a small on-chip memory with very low latency.
Taking advantage of this memory can deliver substantial performance improvements. One common way of doing this is to use shared memory as a softwaremanaged cache to hold frequently reused data. Modifications using shared memory
are shown in Figure A.8.5.
In the context of sparse matrix multiplication, we observe that several rows of
A may use a particular array element x[i]. In many common cases, and particularly
when the matrix has been reordered, the rows using x[i] will be rows near row i.
We can therefore implement a simple caching scheme and expect to achieve some
performance benefit. The block of threads processing rows i through j will load
x[i] through x[j] into its shared memory. We will unroll the multiply_row()
loop and fetch elements of x from the cache whenever possible. The resulting
code is shown in Figure A.8.5. Shared memory can also be used to make other
optimizations, such as fetching Ap[row+1] from an adjacent thread rather than
refetching it from memory.
Because the Tesla architecture provides an explicitly managed on-chip shared
memory, rather than an implicitly active hardware cache, it is fairly common to add
this sort of optimization. Although this can impose some additional development
burden on the programmer, it is relatively minor, and the potential performance
benefits can be substantial. In the example shown above, even this fairly simple
use of shared memory returns a roughly 20% performance improvement on
representative matrices derived from 3D surface meshes. The availability of an
explicitly managed memory in lieu of an implicit cache also has the advantage
that caching and prefetching policies can be specifically tailored to the application
needs.
A.8
Real Stuff: Mapping Applications to GPUs
__global__
void csrmul_cached(unsigned int *Ap, unsigned int *Aj,
float *Av, unsigned int num_rows,
const float *x, float *y)
{
// Cache the rows of x[] corresponding to this block.
__shared__ float cache[blocksize];
unsigned int block_begin = blockIdx.x * blockDim.x;
unsigned int block_end
= block_begin + blockDim.x;
unsigned int row
= block_begin + threadIdx.x;
// Fetch and cache our window of x[].
if( row<num_rows) cache[threadIdx.x] = x[row];
__syncthreads();
if( row<num_rows )
{
unsigned int row_begin = Ap[row];
unsigned int row_end
= Ap[row+1];
float sum = 0, x_j;
for(unsigned int col=row_begin; col<row_end; ++col)
{
unsigned int j = Aj[col];
// Fetch x_j from our cache when possible
if( j>=block_begin && j<block_end )
x_j = cache[j-block_begin];
else
x_j = x[j];
sum += Av[col] * x_j;
}
y[row] = sum;
}
}
FIGURE A.8.5
Shared memory version of sparse matrix-vector multiply.
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Appendix A Graphics and Computing GPUs
These are fairly simple kernels whose purpose is to illustrate basic techniques
in writing CUDA programs, rather than how to achieve maximal performance.
Numerous possible avenues for optimization are available, several of which are
explored by Williams, et al. [2007] on a handful of different multicore architectures.
Nevertheless, it is still instructive to examine the comparative performance of
even these simplistic kernels. On a 2 GHz Intel Core2 Xeon E5335 processor,
the csrmul_serial() kernel runs at roughly 202 million nonzeros processed
per second, for a collection of Laplacian matrices derived from 3D triangulated
surface meshes. Parallelizing this kernel with the parallel_for construct
provided by Intel’s Threading Building Blocks produces parallel speed-ups of 2.0,
2.1, and 2.3 running on two, four, and eight cores of the machine, respectively.
On a GeForce 8800 Ultra, the csrmul_kernel() and csrmul_cached()
kernels achieve processing rates of roughly 772 and 920 million nonzeros per
second, corresponding to parallel speed-ups of 3.8 and 4.6 times over the serial
performance of a single CPU core.
Scan and Reduction
Parallel scan, also known as parallel prefix sum, is one of the most important
building blocks for data-parallel algorithms [Blelloch, 1990]. Given a sequence a
of n elements:
[a0, a1, . . ., an−1]
and a binary associative operator ⊕, the scan function computes the sequence:
scan(a, ⊕) = [a0, (a0 ⊕ a1), . . ., (a0 ⊕ a1 ⊕ . . . ⊕ an−1)]
As an example, if we take ⊕ to be the usual addition operator, then applying scan
to the input array
a = [3 1 7 0 4 1 6 3]
will produce the sequence of partial sums:
scan(a, +) = [3 4 11 11 15 16 22 25]
This scan operator is an inclusive scan, in the sense that element i of the output
sequence incorporates element a i of the input. Incorporating only previous
elements would yield an exclusive scan operator, also known as a prefix-sum
operation.
The serial implementation of this operation is extremely simple. It is simply a
loop that iterates once over the entire sequence, as shown in Figure A.8.6.
At first glance, it might appear that this operation is inherently serial. However,
it can actually be implemented in parallel efficiently. The key observation is that
A.8
Real Stuff: Mapping Applications to GPUs
template<class T>
__host__ T plus_scan(T *x, unsigned int n)
{
for(unsigned int i=1; i<n; ++i)
x[i] = x[i-1] + x[i];
}
FIGURE A.8.6
Template for serial plus-scan.
template<class T>
__device__ T plus_scan(T *x)
{
unsigned int i = threadIdx.x;
unsigned int n = blockDim.x;
for(unsigned int offset=1; offset<n; offset *= 2)
{
T t;
if(i>=offset) t = x[i-offset];
__syncthreads();
if(i>=offset) x[i] = t + x[i];
__syncthreads();
}
return x[i];
}
FIGURE A.8.7
CUDA template for parallel plus-scan.
because addition is associative, we are free to change the order in which elements are
added together. For instance, we can imagine adding pairs of consecutive elements
in parallel, and then adding these partial sums, and so on.
One simple scheme for doing this is from Hillis and Steele [1989]. An
implementation of their algorithm in CUDA is shown in Figure A.8.7. It assumes
that the input array x[] contains exactly one element per thread of the thread
block. It performs log2 n iterations of a loop collecting partial sums together.
To understand the action of this loop, consider Figure A.8.8, which illustrates
the simple case for n = 8 threads and elements. Each level of the diagram represents
one step of the loop. The lines indicate the location from which the data is being
fetched. For each element of the output (i.e., the final row of the diagram) we are
building a summation tree over the input elements. The edges highlighted in blue
show the form of this summation tree for the final element. The leaves of this tree
are all the initial elements. Tracing back from any output element shows that it
incorporates all input values up to and including itself.
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Appendix A Graphics and Computing GPUs
x[0]
x[1]
x[2]
x[3]
x[4]
x[5]
x[6]
x[7]
x[0]
x[1]
x[2]
x[3]
x[4]
x[5]
x[6]
x[7]
x[i] + = x[i –1];
x[0]
x[1]
x[2]
x[3]
x[4]
x[5]
x[6]
x[7]
x[i] + = x[i –2];
x[0]
x[1]
x[2]
x[3]
x[4]
x[5]
x[6]
x[7]
x[i] + = x[i –4];
FIGURE A.8.8 Tree-based parallel scan data references.
While simple, this algorithm is not as efficient as we would like. Examining
the serial implementation, we see that it performs O(n) additions. The parallel
implementation, in contrast, performs O(n log n) additions. For this reason, it
is not work efficient, since it does more work than the serial implementation to
compute the same result. Fortunately, there are other techniques for implementing
scan that are work efficient. Details on more efficient implementation techniques
and the extension of this per-block procedure to multiblock arrays are provided by
Sengupta, et al. [2007].
In some instances, we may only be interested in computing the sum of all elements
in an array, rather than the sequence of all prefix sums returned by scan. This is the
parallel reduction problem. We could simply use a scan algorithm to perform this
computation, but reduction can generally be implemented more efficiently than scan.
Figure A.8.9 shows the code for computing a reduction using addition. In
this example, each thread simply loads one element of the input sequence (i.e.,
it initially sums a subsequence of length 1). At the end of the reduction, we want
thread 0 to hold the sum of all elements initially loaded by the threads of its block.
The loop in this kernel implicitly builds a summation tree over the input elements,
much like the scan algorithm above.
At the end of this loop, thread 0 holds the sum of all the values loaded by this
block. If we want the final value of the location pointed to by total to contain the
total of all elements in the array, we must combine the partial sums of all the blocks
in the grid. One strategy to do this would be to have each block write its partial
sum into a second array and then launch the reduction kernel again, repeating
the process until we had reduced the sequence to a single value. A more attractive
alternative supported by the Tesla GPU architecture is to use the atomicAdd()
A.8
Real Stuff: Mapping Applications to GPUs
__global__
void plus_reduce(int *input, unsigned int N, int *total)
{
unsigned int tid = threadIdx.x;
unsigned int i
= blockIdx.x*blockDim.x + threadIdx.x;
// Each block loads its elements into shared memory, padding
// with 0 if N is not a multiple of blocksize
__shared__ int x[blocksize];
x[tid] = (i<N) ? input[i] : 0;
__syncthreads();
// Every thread now holds 1 input value in x[]
//
// Build summation tree over elements.
for(int s=blockDim.x/2; s>0; s=s/2)
{
if(tid < s) x[tid] += x[tid + s];
__syncthreads();
}
// Thread 0 now holds the sum of all input values
// to this block. Have it add that sum to the running total
if( tid == 0 ) atomicAdd(total, x[tid]);
}
FIGURE A.8.9
CUDA implementation of plus-reduction.
primitive, an efficient atomic read-modify-write primitive supported by the
memory subsystem. This eliminates the need for additional temporary arrays and
repeated kernel launches.
Parallel reduction is an essential primitive for parallel programming and
highlights the importance of per-block shared memory and low-cost barriers in
making cooperation among threads efficient. This degree of data shuffling among
threads would be prohibitively expensive if done in off-chip global memory.
Radix Sort
One important application of scan primitives is in the implementation of sorting
routines. The code in Figure A.8.10 implements a radix sort of integers across a
single thread block. It accepts as input an array values containing one 32-bit
integer for each thread of the block. For efficiency, this array should be stored in
per-block shared memory, but this is not required for the sort to behave correctly.
This is a fairly simple implementation of radix sort. It assumes the availability
of a procedure partition_by_bit() that will partition the given array such that
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Appendix A Graphics and Computing GPUs
__device__ void radix_sort(unsigned int *values)
{
for(int bit=0; bit<32; ++bit)
{
partition_by_bit(values, bit);
__syncthreads();
}
}
FIGURE A.8.10 CUDA code for radix sort.
__device__ void partition_by_bit(unsigned int *values,
unsigned int bit)
{
unsigned int i
= threadIdx.x;
unsigned int size = blockDim.x;
unsigned int x_i = values[i];
unsigned int p_i = (x_i >> bit) & 1;
values[i] = p_i;
__syncthreads();
// Compute number of T bits up to and including p_i.
// Record the total number of F bits as well.
unsigned int T_before = plus_scan(values);
unsigned int T_total = values[size-1];
unsigned int F_total = size - T_total;
__syncthreads();
// Write every x_i to its proper place
if( p_i )
values[T_before-1 + F_total] = x_i;
else
values[i - T_before] = x_i;
}
FIGURE A.8.11 CUDA code to partition data on a bit-by-bit basis, as part of radix sort.
all values with a 0 in the designated bit will come before all values with a 1 in that
bit. To produce the correct output, this partitioning must be stable.
Implementing the partitioning procedure is a simple application of scan. Thread
i holds the value xi and must calculate the correct output index at which to write
this value. To do so, it needs to calculate (1) the number of threads j < i for which
the designated bit is 1 and (2) the total number of bits for which the designated bit
is 0. The CUDA code for partition_by_bit() is shown in Figure A.8.11.
A.8
Real Stuff: Mapping Applications to GPUs
A similar strategy can be applied for implementing a radix sort kernel that sorts
an array of large length, rather than just a one-block array. The fundamental step
remains the scan procedure, although when the computation is partitioned across
multiple kernels, we must double-buffer the array of values rather than doing the
partitioning in place. Details on performing radix sorts on large arrays efficiently
are provided by Satish, Harris, and Garland [2008].
N-Body Applications on a GPU1
Nyland, Harris, and Prins [2007] describe a simple yet useful computational
kernel with excellent GPU performance—the all-pairs N-body algorithm. It is a
time-consuming component of many scientific applications. N-body simulations
calculate the evolution of a system of bodies in which each body continuously
interacts with every other body. One example is an astrophysical simulation in
which each body represents an individual star, and the bodies gravitationally attract
each other. Other examples are protein folding, where N-body simulation is used
to calculate electrostatic and van der Waals forces; turbulent fluid flow simulation;
and global illumination in computer graphics.
The all-pairs N-body algorithm calculates the total force on each body in the
system by computing each pair-wise force in the system, summing for each body.
Many scientists consider this method to be the most accurate, with the only loss of
precision coming from the floating-point hardware operations. The drawback is its
O(n2) computational complexity, which is far too large for systems with more than
106 bodies. To overcome this high cost, several simplifications have been proposed
to yield O(n log n) and O(n) algorithms; examples are the Barnes-Hut algorithm,
the Fast Multipole Method and Particle-Mesh-Ewald summation. All of the fast
methods still rely on the all-pairs method as a kernel for accurate computation of
short-range forces; thus it continues to be important.
N-Body Mathematics
For gravitational simulation, calculate the body-body force using elementary
physics. Between two bodies indexed by i and j, the 3D force vector is:
mimj
rij
||rij||
||rij||
fij = G 2 × The force magnitude is calculated in the left term, while the direction is computed
in the right (unit vector pointing from one body to the other).
Given a list of interacting bodies (an entire system or a subset), the calculation
is simple: for all pairs of interactions, compute the force and sum for each body.
Once the total forces are calculated, they are used to update each body’s position
and velocity, based on the previous position and velocity. The calculation of the
forces has complexity O(n2), while the update is O(n).
1
Adapted from Nyland, Harris and Prins [2007], “Fast N-Body Simulation with CUDA,”
Chapter 31 of GPU Gems 3.
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Appendix A Graphics and Computing GPUs
The serial force-calculation code uses two nested for-loops iterating over pairs
of bodies. The outer loop selects the body for which the total force is being calculated, and the inner loop iterates over all the bodies. The inner loop calls a function
that computes the pair-wise force, then adds the force into a running sum.
To compute the forces in parallel, we assign one thread to each body, since the
calculation of force on each body is independent of the calculation on all other
bodies. Once all of the forces are computed, the positions and velocities of the
bodies can be updated.
The code for the serial and parallel versions is shown in Figure A.8.12 and
Figure A.8.13. The serial version has two nested for-loops. The conversion to CUDA,
like many other examples, converts the serial outer loop to a per-thread kernel
where each thread computes the total force on a single body. The CUDA kernel
computes a global thread ID for each thread, replacing the iterator variable of the
serial outer loop. Both kernels finish by storing the total acceleration in a global
array used to compute the new position and velocity values in a subsequent step.
void accel_on_all_bodies()
{
int i, j;
float3 acc(0.0f, 0.0f, 0.0f);
for (i = 0;
for (j =
acc =
}
accel[i]
}
i < N; i++) {
0; j < N; j++) {
body_body_interaction(acc, body[i], body[j]);
= acc;
}
FIGURE A.8.12 Serial code to compute all pair-wise forces on N bodies.
__global__ void accel_on_one_body()
{
int i = threadIdx.x + blockDim.x * blockIdx.x;
int j;
float3 acc(0.0f, 0.0f, 0.0f);
for (j = 0; j < N; j++) {
acc = body_body_interaction(acc, body[i], body[j]);
}
accel[i] = acc;
}
FIGURE A.8.13 CUDA thread code to compute the total force on a single body.
A.8
Real Stuff: Mapping Applications to GPUs
A-67
The outer loop is replaced by a CUDA kernel grid that launches N threads, one for
each body.
Optimization for GPU Execution
The CUDA code shown is functionally correct, but is not efficient, as it ignores
key architectural features. Better performance can be achieved with three main
optimizations. First, shared memory can be used to avoid identical memory reads
between threads. Second, using multiple threads per body improves performance
for small values of N. Third, loop unrolling reduces loop overhead.
Using Shared memory
Shared memory can hold a subset of body positions, much like a cache, eliminating
redundant global memory requests between threads. We optimize the code shown
above to have each of p threads in a thread-block load one position into shared
memory (for a total of p positions). Once all the threads have loaded a value into
shared memory, ensured by __syncthreads(), each thread can then perform p
interactions (using the data in shared memory). This is repeated N/p times to
complete the force calculation for each body, which reduces the number of requests
to memory by a factor of p (typically in the range 32–128).
The function called accel_on_one_body() requires a few changes to support
this optimization. The modified code is shown in Figure A.8.14.
__shared__ float4 shPosition[256];
…
__global__ void accel_on_one_body()
{
int i = threadIdx.x + blockDim.x * blockIdx.x;
int j, k;
int p = blockDim.x;
float3 acc(0.0f, 0.0f, 0.0f);
float4 myBody = body[i];
for (j = 0; j < N; j += p) { // Outer loops jumps by p each time
shPosition[threadIdx.x] = body[j+threadIdx.x];
__syncthreads();
for (k = 0; k < p; k++) { // Inner loop accesses p positions
acc = body_body_interaction(acc, myBody, shPosition[k]);
}
__syncthreads();
}
accel[i] = acc;
}
FIGURE A.8.14
CUDA code to compute the total force on each body, using shared memory to improve performance.
Appendix A Graphics and Computing GPUs
The loop that formerly iterated over all bodies now jumps by the block
dimension p. Each iteration of the outer loop loads p successive positions into
shared memory (one position per thread). The threads synchronize, and then
p force calculations are computed by each thread. A second synchronization is
required to ensure that new values are not loaded into shared memory prior to all
threads completing the force calculations with the current data.
Using shared memory reduces the memory bandwidth required to less than
10% of the total bandwidth that the GPU can sustain (using less than 5 GB/s).
This optimization keeps the application busy performing computation rather than
waiting on memory accesses, as it would have without the use of shared memory.
The performance for varying values of N is shown in Figure A.8.15.
Using Multiple Threads per Body
Figure A.8.15 shows performance degradation for problems with small values of N
(N < 4096) on the GeForce 8800 GTX. Many research efforts that rely on N-body
calculations focus on small N (for long simulation times), making it a target of
our optimization efforts. Our presumption to explain the lower performance was
that there was simply not enough work to keep the GPU busy when N is small.
The solution is to allocate more threads per body. We change the thread-block
dimensions from (p, 1, 1) to (p, q, 1), where q threads divide the work of a single body
into equal parts. By allocating the additional threads within the same thread block,
partial results can be stored in shared memory. When all the force calculations are
N-Body Performance on GPUs
250
200
GFLOPS
150
1 thread, 8800
2 threads, 8800
100
4 threads, 8800
1 thread, 9600
50
2 threads, 9600
4 threads, 9600
32768
24576
16384
12288
8192
6144
4096
3072
2048
1536
1024
768
0
512
A-68
Number of Bodies
FIGURE A.8.15 Performance measurements of the N-body application on a GeForce 8800
GTX and a GeForce 9600. The 8800 has 128 stream processors at 1.35 GHz, while the 9600 has 64 at
0.80 GHz (about 30% of the 8800). The peak performance is 242 GFLOPS. For a GPU with more processors,
the problem needs to be bigger to achieve full performance (the 9600 peak is around 2048 bodies, while the
8800 doesn’t reach its peak until 16,384 bodies). For small N, more than one thread per body can significantly
improve performance, but eventually incurs a performance penalty as N grows.
A.8
Real Stuff: Mapping Applications to GPUs
done, the q partial results can be collected and summed to compute the final result.
Using two or four threads per body leads to large improvements for small N.
As an example, the performance on the 8800 GTX jumps by 110% when N = 1024
(one thread achieves 90 GFLOPS, where four achieve 190 GFLOPS). Performance
degrades slightly on large N, so we only use this optimization for N smaller than
4096. The performance increases are shown in Figure A.8.15 for a GPU with 128
processors and a smaller GPU with 64 processors clocked at two-thirds the speed.
Performance Comparison
The performance of the N-body code is shown in Figure A.8.15 and Figure A.8.16.
In Figure A.8.15, performance of high- and medium-performance GPUs is shown,
along with the performance improvements achieved by using multiple threads
per body. The performance on the faster GPU ranges from 90 to just under 250
GFLOPS.
Figure A.8.16 shows nearly identical code (C++ versus CUDA) running on Intel
Core2 CPUs. The CPU performance is about 1% of the GPU, in the range of 0.2 to
2 GFLOPS, remaining nearly constant over the wide range of problem sizes.
N-Body Performance on Intel CPUs
GFLOPS
2
1.8
1.6
1.4
1.2
1
0.8
0.6
T2400
E8200
0.4
0.2
0
X9775
32768
24576
16384
12288
8192
6144
4096
3072
2048
1536
1024
768
512
X9775-Cuda
Number of Bodies
FIGURE A.8.16 Performance measurements on the N-body code on a CPU. The graph shows
single precision N-body performance using Intel Core2 CPUs, denoted by their CPU model number. Note
the dramatic reduction in GFLOPS performance (shown in GFLOPS on the y-axis), demonstrating how
much faster the GPU is compared to the CPU. The performance on the CPU is generally independent of
problem size, except for an anomalously low performance when N=16,384 on the X9775 CPU. The graph
also shows the results of running the CUDA version of the code (using the CUDA-for-CPU compiler) on a
single CPU core, where it outperforms the C++ code by 24%. As a programming language, CUDA exposes
parallelism and locality that a compiler can exploit. The Intel CPUs are a 3.2 GHz Extreme X9775 (code
named “Penryn”), a 2.66 GHz E8200 (code named “Wolfdale”), a desktop, pre-Penryn CPU, and a 1.83 GHz
T2400 (code named “Yonah”), a 2007 laptop CPU. The Penryn version of the Core 2 architecture is particularly interesting for N-body calculations with its 4-bit divider, allowing division and square root operations
to execute four times faster than previous Intel CPUs.
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Appendix A Graphics and Computing GPUs
The graph also shows the results of compiling the CUDA version of the code
for a CPU, where the performance improves by 24%. CUDA, as a programming
language, exposes parallelism, allowing the compiler to make better use of the SSE
vector unit on a single core. The CUDA version of the N-body code naturally maps
to multicore CPUs as well (with grids of blocks), where it achieves nearly perfect
scaling on an eight-core system with N = 4096 (ratios of 2.0, 3.97, and 7.94 on two,
four, and eight cores, respectively).
Results
With a modest effort, we developed a computational kernel that improves GPU
performance over multicore CPUs by a factor of up to 157. Execution time for
the N-body code running on a recent CPU from Intel (Penryn X9775 at 3.2 GHz,
single core) took more than 3 seconds per frame to run the same code that runs at a
44 Hz frame rate on a GeForce 8800 GPU. On pre-Penryn CPUs, the code requires
6–16 seconds, and on older Core2 processors and Pentium IV processor, the time
is about 25 seconds. We must divide the apparent increase in performance in half,
as the CPU requires only half as many calculations to compute the same result
(using the optimization that the forces on a pair of bodies are equal in strength and
opposite in direction).
How can the GPU speed up the code by such a large amount? The answer
requires inspecting architectural details. The pair-wise force calculation requires
20 floating-point operations, comprised mostly of addition and multiplication
instructions (some of which can be combined using a multiply-add instruction),
but there are also division and square root instructions for vector normalization. Intel CPUs take many cycles for single precision division and square root
instructions,2 although this has improved in the latest Penryn CPU family with
its faster 4-bit divider.3 Additionally, the limitations in register capacity leads to
many MOV instructions in the x86 code (presumably to/from L1 cache). In contrast, the GeForce 8800 executes a reciprocal square-root thread instruction in four
clocks; see Section A.6 for special function accuracy. It has a larger register file (per
thread) and shared memory that can be accessed as an instruction operand. Finally,
the CUDA compiler emits 15 instructions for one iteration of the loop, compared
with more than 40 instructions from a variety of x86 CPU compilers. Greater
parallelism, faster execution of complex instructions, more register space, and an
efficient compiler all combine to explain the dramatic performance improvement
of the N-body code between the CPU and the GPU.
2
The x86 SSE instructions reciprocal-square-root (RSQRT*) and reciprocal (RCP*) were not
considered, as their accuracy is too low to be comparable.
3
Intel Corporation, Intel 64 and IA-32 Architectures Optimization Reference Manual. November
2007. Order Number: 248966-016. Also available at www3.intel.com/design/processor/manuals/
248966.pdf.
A.8
Real Stuff: Mapping Applications to GPUs
On a GeForce 8800, the all-pairs N-body algorithm delivers more than 240
GFLOPS of performance, compared to less than 2 GFLOPS on recent sequential
processors. Compiling and executing the CUDA version of the code on a CPU
demonstrates that the problem scales well to multicore CPUs, but is still significantly
slower than a single GPU.
We coupled the GPU N-body simulation with a graphical display of the motion,
and can interactively display 16K bodies interacting at 44 frames per second.
This allows astrophysical and biophysical events to be displayed and navigated at
interactive rates. Additionally, we can parameterize many settings, such as noise
reduction, damping, and integration techniques, immediately displaying their
effects on the dynamics of the system. This provides scientists with stunning
visual imagery, boosting their insights on otherwise invisible systems (too large
or small, too fast or too slow), allowing them to create better models of physical
phenomena.
Figure A.8.17 shows a time-series display of an astrophysical simulation
of 16K bodies, with each body acting as a galaxy. The initial configuration is a
FIGURE A.8.17
12 images captured during the evolution of an N-body system with 16,384 bodies.
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Appendix A Graphics and Computing GPUs
spherical shell of bodies rotating about the z-axis. One phenomenon of interest
to astrophysicists is the clustering that occurs, along with the merging of galaxies
over time. For the interested reader, the CUDA code for this application is available
in the CUDA SDK from www.nvidia.com/CUDA.
A.9
Fallacies and Pitfalls
GPUs have evolved and changed so rapidly that many fallacies and pitfalls have
arisen. We cover a few here.
Fallacy: GPUs are just SIMD vector multiprocessors. It is easy to draw the false
conclusion that GPUs are simply SIMD vector multiprocessors. GPUs do have a
SPMD-style programming model, in that a programmer can write a single program that is executed in multiple thread instances with multiple data. The execution of these threads is not purely SIMD or vector, however; it is single-instruction
multiple-thread (SIMT), described in Section A.4. Each GPU thread has its own
scalar registers, thread private memory, thread execution state, thread ID, independent execution and branch path, and effective program counter, and can address
memory independently. Although a group of threads (e.g., a warp of 32 threads)
executes more efficiently when the PCs for the threads are the same, this is not
necessary. So, the multiprocessors are not purely SIMD. The thread execution
model is MIMD with barrier synchronization and SIMT optimizations. Execution
is more efficient if individual thread load/store memory accesses can be coalesced
into block accesses, as well. However, this is not strictly necessary. In a purely SIMD
vector architecture, memory/register accesses for different threads must be aligned
in a regular vector pattern. A GPU has no such restriction for register or memory accesses; however, execution is more efficient if warps of threads access local
blocks of data.
In a further departure from a pure SIMD model, an SIMT GPU can execute
more than one warp of threads concurrently. In graphics applications, there may
be multiple groups of vertex programs, pixel programs, and geometry programs
running in the multiprocessor array concurrently. Computing programs may also
execute different programs concurrently in different warps.
Fallacy: GPU performance cannot grow faster than Moore’s law. Moore’s law
is simply a rate. It is not a “speed of light” limit for any other rate. Moore’s law
describes an expectation that over time, as semiconductor technology advances
and transistors become smaller, the manufacturing cost per transistor will decline
A.9
Fallacies and Pitfalls
exponentially. Put another way, given a constant manufacturing cost, the number
of transistors will increase exponentially. Gordon Moore [1965] predicted that
this progression would provide roughly two times the number of transistors for
the same manufacturing cost every year, and later revised it to doubling every
two years. Although Moore made the initial prediction in 1965 when there were
just 50 components per integrated circuit, it has proved remarkably consistent. The
reduction of transistor size has historically had other benefits, such as lower power
per transistor and faster clock speeds at constant power.
This increasing bounty of transistors is used by chip architects to build processors, memory, and other components. For some time, CPU designers have used
the extra transistors to increase processor performance at a rate similar to Moore’s
law, so much so that many people think that processor performance growth of two
times every 18–24 months is Moore’s law. In fact, it is not.
Microprocessor designers spend some of the new transistors on processor cores,
improving the architecture and design, and pipelining for more clock speed. The
rest of the new transistors are used for providing more cache, to make memory
access faster. In contrast, GPU designers use almost none of the new transistors to
provide more cache; most of the transistors are used for improving the processor
cores and adding more processor cores.
GPUs get faster by four mechanisms. First, GPU designers reap the Moore’s law
bounty directly by applying exponentially more transistors to building more parallel,
and thus faster, processors. Second, GPU designers can improve on the architecture
over time, increasing the efficiency of the processing. Third, Moore’s law assumes
constant cost, so the Moore’s law rate can clearly be exceeded by spending more for
larger chips with more transistors. Fourth, GPU memory systems have increased
their effective bandwidth at a pace nearly comparable to the processing rate, by
using faster memories, wider memories, data compression, and better caches. The
combination of these four approaches has historically allowed GPU performance
to double regularly, roughly every 12 to 18 months. This rate, exceeding the rate
of Moore’s law, has been demonstrated on graphics applications for approximately
ten years and shows no sign of significant slowdown. The most challenging rate
limiter appears to be the memory system, but competitive innovation is advancing
that rapidly too.
Fallacy: GPUs only render 3D graphics; they can’t do general computation.
GPUs are built to render 3D graphics as well as 2D graphics and video. To meet
the demands of graphics software developers as expressed in the interfaces and
performance/feature requirements of the graphics APIs, GPUs have become massively parallel programmable floating-point processors. In the graphics domain,
these processors are programmed through the graphics APIs and with arcane
graphics programming languages (GLSL, Cg, and HLSL, in OpenGL and Direct3D).
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Appendix A Graphics and Computing GPUs
However, there is nothing preventing GPU architects from exposing the parallel
processor cores to programmers without the graphics API or the arcane graphics
languages.
In fact, the Tesla architecture family of GPUs exposes the processors through
a software environment known as CUDA, which allows programmers to develop
general application programs using the C language and soon C++. GPUs are
Turing-complete processors, so they can run any program that a CPU can run,
although perhaps less well. And perhaps faster.
Fallacy: GPUs cannot run double precision floating-point programs fast. In the
past, GPUs could not run double precision floating-point programs at all, except
through software emulation. And that’s not very fast at all. GPUs have made the
progression from indexed arithmetic representation (lookup tables for colors) to
8-bit integers per color component, to fixed-point arithmetic, to single precision
floating-point, and recently added double precision. Modern GPUs perform
virtually all calculations in single precision IEEE floating-point arithmetic, and are
beginning to use double precision in addition.
For a small additional cost, a GPU can support double precision floating-point
as well as single precision floating-point. Today, double precision runs more slowly
than the single precision speed, about five to ten times slower. For incremental
additional cost, double precision performance can be increased relative to single
precision in stages, as more applications demand it.
Fallacy: GPUs don’t do floating-point correctly. GPUs, at least in the Tesla architecture family of processors, perform single precision floating-point processing at
a level prescribed by the IEEE 754 floating-point standard. So, in terms of accuracy,
GPUs are the equal of any other IEEE 754–compliant processors.
Today, GPUs do not implement some of the specific features described in the
standard, such as handling denormalized numbers and providing precise floatingpoint exceptions. However, the recently introduced Tesla T10P GPU provides full
IEEE rounding, fused-multiply-add, and denormalized number support for double
precision.
Pitfall: Just use more threads to cover longer memory latencies. CPU cores are
typically designed to run a single thread at full speed. To run at full speed, every
instruction and its data need to be available when it is time for that instruction to
run. If the next instruction is not ready or the data required for that instruction is
not available, the instruction cannot run and the processor stalls. External memory
is distant from the processor, so it takes many cycles of wasted execution to fetch
data from memory. Consequently, CPUs require large local caches to keep running
A.9
Fallacies and Pitfalls
without stalling. Memory latency is long, so it is avoided by striving to run in the
cache. At some point, program working set demands may be larger than any cache.
Some CPUs have used multithreading to tolerate latency, but the number of threads
per core has generally been limited to a small number.
The GPU strategy is different. GPU cores are designed to run many threads
concurrently, but only one instruction from any thread at a time. Another way to
say this is that a GPU runs each thread slowly, but in aggregate runs the threads
efficiently. Each thread can tolerate some amount of memory latency, because
other threads can run.
The downside of this is that multiple—many multiple threads—are required
to cover the memory latency. In addition, if memory accesses are scattered or not
correlated among threads, the memory system will get progressively slower in
responding to each individual request. Eventually, even the multiple threads will
not be able to cover the latency. So, the pitfall is that for the “just use more threads”
strategy to work for covering latency, you have to have enough threads, and the
threads have to be well-behaved in terms of locality of memory access.
Fallacy: O(n) algorithms are difficult to speed up. No matter how fast the GPU is
at processing data, the steps of transferring data to and from the device may limit
the performance of algorithms with O(n) complexity (with a small amount of work
per datum). The highest transfer rate over the PCIe bus is approximately 48 GB/
second when DMA transfers are used, and slightly less for nonDMA transfers. The
CPU, in contrast, has typical access speeds of 8–12 GB/second to system memory.
Example problems, such as vector addition, will be limited by the transfer of the
inputs to the GPU and the returning output from the computation.
There are three ways to overcome the cost of transferring data. First, try to leave
the data on the GPU for as long as possible, instead of moving the data back and
forth for different steps of a complicated algorithm. CUDA deliberately leaves data
alone in the GPU between launches to support this.
Second, the GPU supports the concurrent operations of copy-in, copy-out and
computation, so data can be streamed in and out of the device while it is computing.
This model is useful for any data stream that can be processed as it arrives. Examples
are video processing, network routing, data compression/decompression, and even
simpler computations such as large vector mathematics.
The third suggestion is to use the CPU and GPU together, improving performance
by assigning a subset of the work to each, treating the system as a heterogeneous
computing platform. The CUDA programming model supports allocation of work
to one or more GPUs along with continued use of the CPU without the use of
threads (via asynchronous GPU functions), so it is relatively simple to keep all
GPUs and a CPU working concurrently to solve problems even faster.
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Appendix A Graphics and Computing GPUs
A.10
Concluding Remarks
GPUs are massively parallel processors and have become widely used, not only
for 3D graphics, but also for many other applications. This wide application
was made possible by the evolution of graphics devices into programmable
processors. The graphics application programming model for GPUs is usually an
API such as DirectXTM or OpenGLTM. For more general-purpose computing, the
CUDA programming model uses an SPMD (single-program multiple data) style,
executing a program with many parallel threads.
GPU parallelism will continue to scale with Moore’s law, mainly by increasing
the number of processors. Only the parallel programming models that can readily
scale to hundreds of processor cores and thousands of threads will be successful
in supporting manycore GPUs and CPUs. Also, only those applications that have
many largely independent parallel tasks will be accelerated by massively parallel
manycore architectures.
Parallel programming models for GPUs are becoming more flexible, for both
graphics and parallel computing. For example, CUDA is evolving rapidly in the
direction of full C/C++ functionality. Graphics APIs and programming models will
likely adapt parallel computing capabilities and models from CUDA. Its SPMDstyle threading model is scalable, and is a convenient, succinct, and easily learned
model for expressing large amounts of parallelism.
Driven by these changes in the programming models, GPU architecture is in
turn becoming more flexible and more programmable. GPU fixed-function units
are becoming accessible from general programs, along the lines of how CUDA
programs already use texture intrinsic functions to perform texture lookups using
the GPU texture instruction and texture unit.
GPU architecture will continue to adapt to the usage patterns of both graphics
and other application programmers. GPUs will continue to expand to include more
processing power through additional processor cores, as well as increasing the thread
and memory bandwidth available for programs. In addition, the programming
models must evolve to include programming heterogeneous manycore systems
including both GPUs and CPUs.
Acknowledgments
This appendix is the work of several authors at NVIDIA. We gratefully acknowledge
the significant contributions of Michael Garland, John Montrym, Doug Voorhies,
Lars Nyland, Erik Lindholm, Paulius Micikevicius, Massimiliano Fatica, Stuart
Oberman, and Vasily Volkov.
A.11
A.11
Historical Perspective and Further Reading
Historical Perspective and Further
Reading
This section, which appears in the Historical Perspective and Further Reading section
of the CD, surveys the history of programmable real-time graphics processing units
(GPUs) from the early 1980s through today as they declined in price by two orders
of magnitude and increased in performance by two orders of magnitude. It traces
the evolution of the GPU from fixed function pipelines to programmable graphics
processors, with perspectives on GPU computing, unified graphics and computing
processors, visual computing, and scalable GPUs.
A-77
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