Operating System Support for Planetary-Scale

Operating System Support for Planetary-Scale
Operating System Support for Planetary-Scale Network Services
Andy Bavier∗ Mic Bowman† Brent Chun† David Culler‡ Scott Karlin∗ Steve Muir∗
Larry Peterson∗ Timothy Roscoe† Tammo Spalink∗ Mike Wawrzoniak∗
∗ Department
of Computer Science
Princeton University
† Intel
Abstract
PlanetLab is a geographically distributed overlay network
designed to support the deployment and evaluation of
planetary-scale network services. Two high-level goals shape
its design. First, to enable a large research community to
share the infrastructure, PlanetLab provides distributed virtualization, whereby each service runs in an isolated slice of
PlanetLab’s global resources. Second, to support competition among multiple network services, PlanetLab decouples
the operating system running on each node from the networkwide services that define PlanetLab, a principle referred to as
unbundled management. This paper describes how PlanetLab realizes the goals of distributed virtualization and unbundled management, with a focus on the OS running on each
node.
1
Introduction
PlanetLab is a geographically distributed overlay platform
designed to support the deployment and evaluation of
planetary-scale network services [30]. It currently includes
over 350 machines spanning 150 sites and 20 countries. It
supports over 450 research projects focused on a wide range
of services, including file sharing and network-embedded
storage [11, 22, 35], content distribution networks [39], routing and multicast overlays [1, 8], QoS overlays [38], scalable
object location services [2, 33, 34, 37], anomaly detection
mechanisms [9], and network measurement tools [36].
As a distributed system, PlanetLab is characterized by a
unique set of relationships between principals—e.g., users,
administrators, researchers, service providers—which make
the design requirements for its operating system different
from traditional hosting services or timesharing systems.
The first relationship is between PlanetLab as an organization, and the institutions that own and host PlanetLab nodes:
the former has administrative control over the nodes, but local sites also need to enforce policies about how the nodes
are used, and the kinds and quantity of network traffic the
Research
‡ Computer
Science Division
University of California, Berkeley
nodes can generate. This implies a need to share control of
PlanetLab nodes.
The second relationship is between PlanetLab and
its users, currently researchers evaluating and deploying
planetary-scale services. Researchers must have access to
the platform, which implies a distributed set of machines that
must be shared in a way they will find useful. A PlanetLab
“account”, together with associated resources, must therefore
span multiple machines. We call this abstraction a slice, and
implement it using a technique called distributed virtualization.
A third relationship exists between PlanetLab and those researchers contributing to the system by designing and building infrastructure services, that is, services that contribute to
the running of the platform as opposed to being merely applications on it. Not only must each of these services run in a
slice, but PlanetLab must support multiple, parallel services
with similar functions developed by different groups. We call
this principle unbundled management, and it imposes its own
requirements on the system.
Finally, PlanetLab exists in relation to the rest of the Internet. Experience shows that the experimental networking performed on PlanetLab can easily impact many external sites’
intrusion detection and vulnerability scanners. This leads
to requirements for policies limiting what traffic PlanetLab
users can send to the rest of the Internet, and a way for concerned outside individuals to find out exactly why they are
seeing unusual traffic from PlanetLab. The rest of the Internet needs to feel safe from PlanetLab.
The contribution of this paper is to describe in more detail
the requirements that result from these relationships, and how
PlanetLab fulfills them using a synthesis of operating systems techniques. This contribution is partly one of design because PlanetLab is a work-in-progress and only time will tell
what infrastructure services will evolve to give it fuller definition. At the same time, however, this design is largely the
product of our experience having hundreds of users stressing PlanetLab since the platform became operational in July
2002.
2
Requirements
the performance of another on the same node. Moreover, certain slices must be able to request a minimal
resource level, and in return, receive (soft) real-time performance guarantees.
This section defines distributed virtualization and unbundled
management, and identifies the requirements each places on
PlanetLab’s design.
2.1
Distributed Virtualization
PlanetLab services and applications run in a slice of the platform: a set of nodes on which the service receives a fraction
of each node’s resources, in the form of a virtual machine
(VM). Virtualization and virtual machines are, of course,
well-established concepts. What is new in PlanetLab is distributed virtualization: the acquisition of a distributed set of
VMs that are treated as a single, compound entity by the system.
To support this concept, PlanetLab must provide facilities
to create a slice, initialize it with sufficient persistent state
to boot the service or application in question, and bind the
slice to a set of resources on each constituent node. However,
much of a slice’s behavior is left unspecified in the architecture. This includes exactly how a slice is created, which we
discuss in the context of unbundled management, as well as
the programming environment PlanetLab provides. Giving
slices as much latitude as possible in defining a suitable environment means, for example, that the PlanetLab OS does
not provide tunnels that connect the constituent VMs into any
particular overlay configuration, but instead provides an interface that allows each service to define its own topology
on top of the fully-connected Internet. Similarly, PlanetLab
does not prescribe a single language or runtime system, but
instead allows slices to load whatever environments or software packages they need.1
2.1.1
Isolating Slices
PlanetLab must isolate slices from each other, thereby maintaining the illusion that each slice spans a distributed set of
private machines. The same requirement is seen in traditional operating systems, except that in PlanetLab the slice
is a distributed set of VMs rather than a single process or
image. Per-node resource guarantees are also required: for
example, some slices run time-sensitive applications, such as
network measurement services, that have soft real-time constraints reminiscent of those provided by multimedia operating systems. This means three things with respect to the
PlanetLab OS:
• It must allocate and schedule node resources (cycles,
bandwidth, memory, and storage) so that the runtime behavior of one slice on a node does not adversely affect
1 This is not strictly true, as PlanetLab currently provides a Unix API at
the lowest level. Our long-term goal, however, is to decouple those aspects
of the API that are unique to PlanetLab from the underlying programming
environment.
• It must either partition or contextualize the available
name spaces (network addresses, file names, etc.) to
prevent a slice interfering with another, or gaining access to information in another slice. In many cases,
this partitioning and contextualizing must be coordinated over the set of nodes in the system.
• It must provide a stable programming base that cannot
be manipulated by code running in one slice in a way
that negatively affects another slice. In the context of
a Unix- or Windows-like operating system, this means
that a slice cannot be given root or system privilege.
Resource scheduling and VM isolation were recognized as
important issues from the start, but the expectation was that
a “best effort” solution would be sufficient for some time.
Our experience, however, is that excessive loads (especially
near conference deadlines) and volatile performance behavior (due to insufficient isolation) were the dominant problems
in early versions of the system. The lack of isolation has also
led to significant management overhead, as human intervention is required to deal with run-away processes, unbounded
log files, and so on.
2.1.2
Isolating PlanetLab
The PlanetLab OS must also protect the outside world from
slices. PlanetLab nodes are simply machines connected
to the Internet, and as a consequence, buggy or malicious
services running in slices have the potential to affect the
global communications infrastructure. Due to PlanetLab’s
widespread nature and its goal of supporting novel network
services, this impact goes far beyond the reach of an application running on any single computer. This places two requirements on the PlanetLab OS.
• It must thoroughly account resource usage, and make
it possible to place limits on resource consumption so
as to mitigate the damage a service can inflict on the
Internet. Proper accounting is also required to isolate
slices from each other. Here, we are concerned both
with the node’s impact on the hosting site (e.g., how
much network bandwidth it consumes) and remote sites
completely unaffiliated with PlanetLab (e.g., sites that
might be probed from a PlanetLab node). Furthermore,
both the local administrators of a PlanetLab site and
PlanetLab as an organization need to collectively set
these policies for a given node.
• It must make it easy to audit resource usage, so that
actions (rather than just resources) can be accounted
to slices after the fact. This concern about how users
(or their services) affect the outside world is a novel requirement for PlanetLab, unlike traditional timesharing
systems, where the interactions between users and unsuspecting outside entities is inherently rare.
Security was recognized from the start as a critical issue
in the design of PlanetLab. However, effectively limiting and
auditing legitimate users has turned out to be just as significant an issue as securing the OS to prevent malicious users
from hijacking machines. For example, a single PlanetLab
user running TCP throughput experiments on U.C. Berkeley
nodes managed to consume over half of the available bandwidth on the campus gateway over a span of days. Also,
many experiments (e.g., Internet mapping) have triggered
IDS mechanisms, resulting in complaints that have caused
local administrators to pull the plug on nodes. The Internet
has turned out to be unexpectedly sensitive to the kinds of
traffic that experimental planetary-scale services tend to generate.
2.2
Unbundled Management
Planetary-scale services are a relatively recent and ongoing
subject of research; in particular, this includes the services required to manage a global platform such as PlanetLab. Moreover, it is an explicit goal of PlanetLab to allow independent
organizations (in this case, research groups) to deploy alternative services in parallel, allowing users to pick which ones
to use. This applies to application-level services targeted at
end-users, as well as infrastructure services used to manage
and control PlanetLab itself (e.g., slice creation, resource and
topology discovery, performance monitoring, and software
distribution). The key to unbundled management is to allow
parallel infrastructure services to run in their own slices of
PlanetLab and evolve over time.
This is a new twist on the traditional problem of how to
evolve a system, where one generally wants to try a new
version of some service in parallel with an existing version,
and roll back and forth between the two versions. In our
case, multiple competing services are simultaneously evolving. The desire to support unbundled management leads to
two requirements for the PlanetLab OS.
• To minimize the functionality subsumed by the PlanetLab OS—and maximize the functionality running as
services on top of the OS—only local (per-node) abstractions should be directly supported by the OS, allowing all global (network-wide) abstractions to be implemented by infrastructure services.
• To maximize the opportunity for services to compete
with each other on a level playing field, the interface
between the OS and these infrastructure services must
be sharable, and hence, without special privilege. In
other words, rather than have a single privileged application controlling a particular aspect of the OS, the
PlanetLab OS potentially supports many such management services. One implication of this interface being
sharable is that it must be well-defined, explicitly exposing the state of the underlying OS. In contrast, the
interface between an OS and a privileged control program running in user space is often ad hoc since the
control program is, in effect, an extension of the OS that
happens to run in user space.
Of particular note, slice creation is itself implemented as a
service running in its own slice, which leads to the following
additional requirement on the PlanetLab OS:
• It must provide a low-level interface for creating a VM
that can be shared by multiple slice creation services.
It must also host a “bootstrapping” slice creation service to create initial slices, including the slices that other
slice creation services run in.
An important technical issue that will influence how the
slice abstraction evolves is how quickly a network-wide slice
can be instantiated. Applications like the ones listed in the Introduction are relatively long-lived (although possibly modified and restarted frequently), and hence the process of creating the slice in which they run can be a heavy-weight operation. On the other hand, a facility for rapidly establishing
and tearing down a slice (analogous to creating/destroying a
network connection) would lead to slices that are relatively
short-lived, for example, a slice that corresponds to a communication session with a known set of participants. We
evaluate the performance of the current slice creation mechanism in Section 5. It is not yet clear what other slice creation
services the user community will provide, or how they will
utilize the capability to create and destroy slices.
The bottom line is that OS design often faces a tension between implementing functionality in the kernel and running
it in user space, the objective often being to minimize kernel code. Like many VMM architectures, the PlanetLab OS
faces an additional, but analogous, tension between what can
run in a slice or VM, and functionality (such as slice user
authentication) that requires extra privilege or access but is
not part of the kernel. In addition, there is a third aspect to
the problem that is peculiar to PlanetLab: functionality that
can be implemented by parallel, competing subsystems, versus mechanisms which by their very nature can only be implemented once (such as bootstrapping slice creation). The
PlanetLab OS strives to minimize the latter, but there remains
a core of non-kernel functionality that has to be unique on a
node.
2.3
Evolving Architecture
While unbundled management addresses the challenge of
evolving PlanetLab as a whole, there remains the very practi-
Slices (VMs)
VMM: Linux++
Node manager
− resource allocation
− sensors
− auditing
− slice bootstrapping
Local admin
− resource limits
− kill process
Figure 1: PlanetLab Node Architecture
cal issue of evolving the underlying OS that supports unbundled management.
Simply stated, the research community was ready to use
PlanetLab the moment the first machines were deployed.
Waiting for a new OS tailored for broad-coverage services
was not an option, and in any case without first gaining some
experience, no one could fully understand what such a system should look like. Moreover, experience with previous
testbeds strongly suggested two biases of application writers: (1) they are seldom willing to port their applications to
a new API, and (2) they expect a full-featured system rather
than a minimalist API tuned for someone else’s OS research
agenda.
This suggested the strategy of starting with a full-featured
OS—we elected to use Linux due to its widespread use in
the research community—and incrementally transforming it
based on experience. This evolution is guided by the “meta”
architecture depicted in Figure 1.
At the lowest level, each PlanetLab node runs a virtual
machine monitor (VMM) that implements and isolates VMs.
The VMM also defines the API to which services are implemented. PlanetLab currently implements the VMM as a
combination of the Linux kernel and a set of kernel extensions, as outlined in Section 4.
A privileged, “root” VM running on top of the VMM,
called the node manager, monitors and manages all the VMs
on the node. Generally speaking, the node manager enforces
policies on creating VMs and allocating resources to them,
with services interacting with the node manager to create
new VMs rather than directly calling the VMM. Moreover,
all interactions with the node manager are local: only services running in some other VM on the node are allowed to
call the node manager, meaning that remote access to a specific node manager is always indirect through one of the services running on the node. Today, most policy is hard-coded
into the node manager, but we expect that local administrators will eventually be able to configure the policies on their
own nodes. (This is the purpose of the local administrator
VM shown in Figure 1.)
A subset of the services (slices) running on top of the
VMM can be characterized as privileged in some way: they
are allowed to make privileged calls to the node manager
(e.g., to allocate local resources to a VM). We expect all
slices that provide a service to end-users to be unprivileged,
while some infrastructure services may need to run in a privileged slice. To date, three types of infrastructure services
are emerging: (1) brokerage services that are used to acquire
resources and create slices that are bound to them, (2) environment services that are used to initialize and maintain a
slice’s code base, and (3) monitoring services that are used to
both discover the available resources and monitor the health
of running services.
Because we expect new facilities to be incorporated into
the architecture over time, the key question is where any
new functionality should be implemented: in an unprivileged
slice, in a privileged slice, in the node manager, or in the
VMM? Such decisions are guided by the following two principles:
• Each function should be implemented at the “highest”
level possible, that is, running a service in a slice with
limited privileged capabilities is preferred to a slice with
widespread privileges, which in turn is preferred to augmenting the node manager, all of which are preferable
to adding the function to the VMM.
• Privileged slices should be granted the minimal privileges necessary to support the desired behavior. They
should not be granted blanket superuser privileges.
3
Design Alternatives
The PlanetLab OS is a synthesis of existing operating systems abstractions and techniques, applied to the new context of a distributed platform, and motivated by the requirements discussed in the previous section. This section discusses how PlanetLab’s requirements recommend certain approaches over others, and in the process, discusses related
work.
3.1
Node Virtualization
The first challenge of the PlanetLab OS is to provide a virtual
machine abstraction for slices; the question is, at what level?
At one end of the spectrum, full hypervisors like VMware
completely virtualize the physical hardware and thus support
multiple, unmodified operating system binaries. If PlanetLab were to supply this low level of virtualization, each slice
could run its own copy of an OS and have access to all of the
devices and resources made available to it by the hypervisor.
This would allow PlanetLab to support OS kernel research,
as well as provide stronger isolation by removing contention
for OS resources. The cost of this approach is performance:
VMware cannot support the number of simultaneous slices
required by PlanetLab due to the large amount of memory
consumed by each machine image. Thus far, the PlanetLab
community has not required the ability to run multiple operating systems, and so PlanetLab is able to take advantage of
the efficiency of supporting a single OS API.
A slightly higher-level approach is to use paravirtualization, proposed by so-called isolation kernels like Xen [4] and
Denali [41]. Short of full virtualization of the hardware, a
subset of the processor’s instruction set and some specialized
virtual devices form the virtual machine exported to users.
Because the virtual machine is no longer a replica of a physical machine, operating systems must be ported to the new
“architecture”, but this architecture can support virtualization far more efficiently. Paravirtualizing systems are not yet
mature, but if they can be shown to scale, they represent a
promising technology for PlanetLab.
The approach we adopted is to virtualize at the systemcall level, similar to commercial offerings like Ensim [14],
and projects such as User Mode Linux [12], BSD’s Jail [21],
and Linux vservers [25]. Such high-level virtualization adequately supports PlanetLab’s goals of supporting large numbers of overlay services, while providing reasonable assurances of isolation.
3.2
Isolation and Resource Allocation
A second, orthogonal challenge is to isolate virtual machines.
Operating systems with the explicit goal of isolating application performance go back at least as far as the KeyKOS system [18], which provided strict resource accounting between
mutually antagonistic users. More recently, isolation mechanisms have been explored for multimedia support, where
many applications require soft real-time guarantees. Here the
central problem is crosstalk, where contention for a shared
resource (often a server process) prevents the OS from correctly scheduling tasks. This has variously been addressed by
sophisticated accounting across control transfers as in Processor Capacity Reserves [27], scheduling along data paths
as in Scout [28], or entirely restructuring the OS to eliminate
server processes in the data path as in Nemesis [23]. The
PlanetLab OS borrows isolation mechanisms from Scout, but
the key difference is in how these mechanisms are controlled,
since each node runs multiple competing tasks that belong to
a global slice, rather than a purely local set of cooperating
tasks.
The problem of distributed coordination of resources, in
turn, has been explored in the context of Condor [26] and
more recently the Open Grid Services Architecture [16].
However, both these systems are aimed at the execution of
batch computations, rather than the support of long-running
network services. They also seek to define complete architectures within which such computations run. In PlanetLab
the requirements are rather different: the platform must support multiple approaches to creating and binding resources
to slices. To illustrate this distinction, we point out that both
the Globus grid toolkit and the account management system
of the Emulab testbed [42] have been implemented above
PlanetLab, as have more service-oriented frameworks like
SHARP [17].
3.3
Network Virtualization
Having settled on node virtualization at the system call
level, the third challenge is how to virtualize the network.
Vertically-structured operating systems like Exokernel and
Nemesis have explored allowing access to raw network devices by using filters on send and receive [6, 13]. The PlanetLab OS uses a similar approach, providing shared network
access using a “safe” version of the raw socket interface.
Exokernels have traditionally either provided the raw
(physical) device to a single library OS to manage, or controlled sharing of the raw device between library OSes based
on connections. However, in PlanetLab the kernel must take
responsibility for sharing raw access (both reception and
transmission of potentially arbitrary packets) among multiple competing services in a controlled manner according to
some administrative policy. Additionally, it must protect the
surrounding network from buggy and malicious services, an
issue typically ignored by existing systems.
An alternative to sharing and partitioning a single network
address space among all virtual machines is to contextualize
it—that is, we could present each VM with its own local version of the space by moving the demultiplexing to another
level. For instance, we could assign a different IP address
to each VM and allow each to use the entire port space and
manage its own routing table. The problem is that we simply
do not have enough IPv4 addresses available to assign on the
order of 1000 to each node.
3.4
Monitoring
A final, and somewhat new challenge is to support the monitoring and management of a large distributed infrastructure. On the network side, commercial management systems
such as HP OpenView and Micromuse Netcool provide simplified interfaces to routing functionality, service provisioning, and equipment status checks. On the host management
side, systems such as IBM’s Tivoli and Computer Associates’
UniCenter address the corresponding problems of managing
large numbers of desktop and server machines in an enterprise. Both kinds of systems are aimed at single organizations, with well-defined applications and goals, seeking to
manage and control the equipment they own. Managing a
wide-area, evolving, federated system like PlanetLab (or the
Internet as a whole) poses different challenges. Here, we are
pretty much on our own.
4
Planetlab OS
This section defines the PlanetLab OS, the per-node software
upon which the global slice abstraction is built. The PlanetLab OS consists of a Linux 2.4-series kernel with patches for
vservers and hierarchical token bucket packet scheduling; the
SILK (Scout in Linux Kernel) module [5, 32] that provides
CPU scheduling, network accounting, and safe raw sockets;
and the node manager, a trusted domain that contains slice
bootstrapping machinery and node monitoring and management facilities. We describe the functionality provided by
these components and discuss how it is used to implement
slices, focusing on four main areas: the VM abstraction, resource allocation, controlled access to the network, and system monitoring.
4.1
Node Virtualization
A slice corresponds to a distributed set of virtual machines.
Each VM, in turn, is implemented as a vserver [25]. The
vserver mechanism is a patch to the Linux 2.4 kernel that provides multiple, independently managed virtual servers running on a single machine. Vservers are the principal mechanism in PlanetLab for providing virtualization on a single
node, and contextualization of name spaces; e.g., user identifiers and files.
As well as providing security between slices sharing a
node, vservers provide a limited root privilege that allows
a slice to customize its VM as if it was a dedicated machine.
Vservers also correspond to the resource containers used for
isolation, which we discuss in section 4.2.
4.1.1
NetBed [42] and RON [1].
A vserver is initialized with two pieces of persistent state:
a set of SSH keys and a vserver-specific rc.vinit file. The
former allow the owners of the slice to SSH into the vserver,
while the latter serves as a boot script that gets executed each
time the vserver starts running.
Vservers communicate with one another via IP, and not
local sockets or other IPC functions. This strong separation
between slices simplifies resource management and isolation
between vservers, since the interaction between two vservers
is independent of their locations. However, the namespace
of network addresses (IP address and port numbers) is not
contextualized: this would imply either an IP address for
each vserver, or hiding each vserver behind a per-node NAT.
We rejected both these options in favor of slices sharing port
numbers and addresses on a single node.
Interface
Vservers provide virtualization at the system call level by extending the non-reversible isolation provided by chroot for
filesystems to other operating system resources, such as processes and SysV IPC. Processes within a vserver are given
full access to the files, processes, SysV IPC, network interfaces, and accounts that can be named in their containing vserver, and are otherwise denied access to system-wide
operating system resources. Each vserver is given its own
UID/GID namespace, along with a weaker form of root
that provides a local superuser without compromising the security of the underlying machine.
Despite having only a subset of the true superuser’s capabilities, vserver root is still useful in practice. It can modify the vserver’s root filesystem, allowing users to customize
the installed software packages for their vserver. Combined
with per-vserver UID/GID namespaces, it allows vservers
to implement their own account management schemes (e.g.,
by maintaining a per-vserver /etc/passwd and running
sshd on a different TCP port), thereby providing the basis for integration with other wide-area testbeds such as
4.1.2
Implementation
Each vserver on a machine is assigned a unique security context, and each process is associated with a specific vserver
through its security context. A process’s security context
is assigned via a new system call and inherited by the process’s descendants. Isolation between vservers is enforced
at the system call interface by using a combination of security context and UID/GID to check access control privileges
and decide what information should be exposed to a given
process. All of these mechanisms are implemented in the
baseline vserver patch to the kernel. We have implemented
several utilities to simplify creating and destroying vservers,
and to transparently redirect a user into the vserver for his or
her specific slice using SSH.
On PlanetLab, our utilities initialize a vserver by creating
a mirror of a reference root filesystem inside the vserver using hard links and the “immutable” and “immutable invert”
filesystem bits. Next we create two Linux accounts with
login name equal to the slice name, one in the node’s primary vserver and one in the vserver just created, and sharing
a single UID. The default shell for the account in the main
vserver is set to /bin/vsh, a modified bash shell that performs the following four actions upon login: switching to the
slice’s vserver security context, chrooting to the vserver’s
root filesystem, relinquishing a subset of the true superuser’s
capabilities, and redirecting into the other account inside the
vserver. The result of this two-account arrangement is that
users accessing their virtual machines remotely via SSH/SCP
are transparently redirected into the appropriate vserver and
need not modify any of their existing service management
scripts.
By virtualizing above a standard Linux kernel, vservers
achieve substantial sharing of physical memory and disk
space, with no active state needed for idle vservers. For physical memory, savings are accrued by having single copies
of the kernel and daemons, and shared read-only and copy-
on-write memory segments across unrelated vservers. Disk
space sharing is achieved using the filesystem immutable invert bit which allows for a primitive form of copy-on-write
(COW). Using COW on chrooted vserver root filesystems,
vserver disk footprints are just 5.7% of that required with
full copies (Section 5.1). Comparable sharing in a virtual machine monitor or isolation kernel is strictly harder, although
with different isolation guarantees.
PlanetLab’s application of vservers makes extensive use
of the Linux capability mechanism. Capabilities determine
whether privileged operations such as pinning physical memory or rebooting are allowed. In general, the vserver root
account is denied all capabilities that could undermine the
security of the machine (e.g., accessing raw devices) and
granted all other capabilities. However, as discussed in Section 2.3, each PlanetLab node supports two special contexts
with additional capabilities: the node manager and the admin
slice.
The node manager context runs with standard root capabilities and includes the machinery to create a slice, initialize
its state, and assign resources to it; sensors that export information about the node; and a traffic auditing service. The
admin slice provides weaker privileges to site administrators,
giving them a set of tools to manage the nodes without providing full root access. The admin context has a complete
view of the machine and can, for example, cap the node’s total outgoing bandwidth rate, kill arbitrary processes, and run
tcpdump to monitor traffic on the local network.
4.1.3
Discussion
Virtualizing above the kernel has a cost: weaker guarantees
on isolation and challenges for eliminating QoS crosstalk.
Unlike virtual machine monitors and isolation kernels that
provide isolation at a low level, vservers implement isolation
at the system call interface. Hence, a malicious vserver that
exploits a Linux kernel vulnerability might gain control of
the operating system, and hence compromise security of the
machine. We have observed such an incident, in which a subset of PlanetLab nodes were compromised in this way. This
would have been less likely using a lower-level VM monitor. Another potential cost incurred by virtualizing above the
kernel is QoS crosstalk. Eliminating all QoS crosstalk (e.g.,
interactions through the buffer cache) is strictly harder with
vservers. As described in the next section, however, fairly
deep isolation can be achieved.
The combination of vservers and capabilities provides
more flexibility in access control than we currently use in
PlanetLab. For example, sensor slices (see section 4.4) could
be granted access to information sources that cannot otherwise easily be shared among clients. As we gain experience
on the privileges services actually require, extending the set
of Linux capabilities is a natural path toward exposing privileged operations in a controlled way.
4.2
Isolation and Resource Allocation
A key feature of slices is the isolation they provide between
services. Early experience with PlanetLab illustrates the
need for slice isolation. For example, we have seen slices
acquire all available file descriptors on several nodes, preventing other slices from using the disk or network; routinely
fill all available disk capacity with unbounded event logging;
and consume 100% of the CPU on 50 nodes by running an infinite loop. Isolating slices is necessary to make the platform
useful.
The node manager provides a low-level interface for obtaining resources on a node and binding them to a local VM
that belongs to some slice. The node manager does not make
any policy decisions regarding how resources are allocated,
nor is it remotely accessible. Instead, a bootstrap brokerage
service running in a privileged slice implements the resource
allocation policy. This policy includes how many resources
to allocate to slices that run other brokerage services; such
services are then free to redistribute those resources to still
other slices. Note that resource allocation is largely controlled by a central policy in the current system, although we
expect it to eventually be defined by the node owner’s local
administrator. Today, the only resource-related policy set by
the local node administrator is an upper bound on the total
outgoing bandwidth that may be consumed by the node.
4.2.1
Interface
The node manager denotes the right to use a set of node resources as a resource capability (rcap)—a 128-bit opaque
value, the knowledge of which provides access to the associated resources. The node manager provides privileged slices
with the following operation to create a resource capability:
rcap ← acquire(rspec)
This operation takes a resource specification (rspec) as an
argument, and returns an rcap should there by sufficient resources on the node to satisfy the rspec. Each node manager
tracks both the set of resources available on the node, and the
mapping between committed resources and the corresponding rcaps; i.e., the rcap serves as an index into a table of
rspecs.
The rspec describes a slice’s privileges and resource reservations over time. Each rspec consists of a list of reservations for physical resources (e.g., CPU cycles, link bandwidth, disk capacity), limits on logical resource usage (e.g.,
file descriptors), assignments of shared name spaces (e.g.,
TCP and UDP port numbers), and other slice privileges (e.g.,
the right to create a virtual machine on the node). The rspec
also specifies the start and end times of the interval over
which these values apply.
Once acquired, rcaps can be passed from one service to
another. The resources associated with the rcap are bound
to a virtual machine (vserver) at slice creation time using the
following operation:
bind(slice name, rcap)
This operation takes a slice identifier and an rcap as arguments, and assuming the rspec associated with the rcap
includes the right to create a virtual machine, creates the
vserver and binds the resources described by the rcap to it.
Note that the node manager supports other operations to manipulate rcaps, as described more fully elsewhere [10].
4.2.2
Implementation
Non-renewable resources, such as memory pages, disk space,
and file descriptors, are isolated using per-slice reservations
and limits. These are implemented by wrapping the appropriate system calls or kernel functions to intercept allocation requests. Each request is either accepted or denied based on the
slice’s current overall usage, and if it is accepted, the slice’s
counter is incremented by the appropriate amount.
For renewable resources such as CPU cycles and link
bandwidth, the OS supports two approaches to providing isolation: fairness and guarantees. Fairness ensures that each of
the N slices running on a node receives no less than 1/N of
the available resources during periods of contention, while
guarantees provide a slice with a reserved amount of the resource (e.g., 1Mbps of link bandwidth). PlanetLab provides
CPU and bandwidth guarantees for slices that request them,
and “fair best effort” service for the rest. In addition to isolating slices from each other, resource limits on outgoing traffic
and CPU usage can protect the rest of the world from PlanetLab.
The Hierarchical Token Bucket (htb) queuing discipline of
the Linux Traffic Control facility (tc) [24] is used to cap the
total outgoing bandwidth of a node, cap per-vserver output,
and to provide bandwidth guarantees and fair service among
vservers. The node administrator configures the root token
bucket with the maximum rate at which the site is willing to
allow traffic to leave the node. At vserver startup, a token
bucket is created that is a child of the root token bucket; if
the service requests a guaranteed bandwidth rate, the token
bucket is configured with this rate, otherwise it is given a
minimal rate (5Kbps) for “fair best effort” service. Packets
sent by a vserver are tagged in the kernel and subsequently
classified to the vserver’s token bucket. The htb queuing discipline then provides each child token bucket with its configured rate, and fairly distributes the excess capacity from
the root to the children that can use it in proportion to their
rates. A bandwidth cap can be placed on each vserver limiting the amount of excess capacity that it is able to use. By
default, the rate of the root token bucket is set at 100Mbps;
each vserver is capped at 10Mbps and given a rate of 5Kbps
for “fair best effort” service.
In addition to this general rate-limiting facility, htb can
also be used to limit the outgoing rate for certain classes of
packets that may raise alarms within the network. For instance, we are able to limit the rate of outgoing pings (as
well as packets containing IP options) to a small number per
second; this simply involves creating additional child token
buckets and classifying outgoing packets so that they end
up in the correct bucket. Identifying potentially troublesome
packets and determining reasonable output rates for them is
a subject of ongoing work.
CPU scheduling is implemented by the SILK kernel module, which leverages Scout [28] to provide vservers with
CPU guarantees and fairness. Replacing Linux’s CPU scheduler was necessary because, while Linux provides approximate fairness between individual processes, it cannot enforce fairness between vservers; nor can it provide guarantees. PlanetLab’s CPU scheduler uses a proportional sharing
(PS) scheduling policy to fairly share the CPU. It incorporates the resource container [3] abstraction and maps each
vserver onto a resource container that possesses some number of shares. Individual processes spawned by the vserver
are all placed within the vserver’s resource container. The result is that the vserver’s set of processes receives a CPU rate
proportional to the vserver’s shares divided by the sum of
shares of all active vservers. For example, if a vserver is assigned 10 shares and the sum of shares of all active vservers
(i.e., vservers that contain a runnable process) is 50, then the
vserver with 10 shares gets 10/50 = 20% of the CPU.
The PS scheduling policy is also used to provide minimum
cycle guarantees by capping the number of shares and using an admission controller to ensure that the cap is not exceeded. The current policy limits the number of outstanding
CPU shares to 1000, meaning that each share is a guarantee for at least 0.1% of the CPU. Additionally, the PlanetLab
CPU scheduler provides a switch to allow a vserver to proportionally share the excess capacity, or to limit it to its guaranteed rate (similar to the Nemesis scheduler [23]). In the
previous example, the vserver with 10 shares received 20%
of the CPU because it was allowed to proportionally share
the excess; with this bit turned off, it would be rate-capped at
10/1000 = 1% of the CPU.
4.2.3
Discussion
We have implemented a centrally-controlled brokerage service, called PlanetLab Central (PLC), that is responsible for
globally creating slices [31]. PLC maintains a database of
principals, slices, resource allocations, and policies on a central server. It exports an interface that includes operations to
create and delete slices; specify a boot script, set of user keys,
and resources to be associated with the slice; and instantiate a
slice on a set of nodes. A per-node component of PLC, called
a resource manager, runs in a privileged slice; it is allowed
to call the acquire operation on each node. The resource
manager on each node periodically communicates with the
central PLC server to obtain policy about what slices can be
created and how many resources are to be bound to each.
PLC allocates resources to participating institutions and
their projects, but it also allocates resources to other brokerage services for redistribution. In this way, PLC serves as a
bootstrap brokerage service, where we expect more sophisticated (and more decentralized) services to evolve over time.
We envision this evolution proceeding along two dimensions.
First, PLC currently allows the resources bound to a slice
to be specified by a simple (share, duration) pair, rather
than exposing the more detailed rspec accepted by the node
manager. The share specifies a relative share of each node’s
CPU and link capacity that the slice may consume, while duration indicates the period of time for which this allocation is
valid. PLC policy specifies how to translate the share value
given by a user into a valid rspec presented to the node manager. Over time we expect PLC to expose more of the rspec
structure directly to users, imposing less policy on resource
allocation decisions. We decided to initially hide the node
manager interface because its semantics are not well-defined
at this point: how to divide resources into allocatable units
is an open problem, and to compensate for this difficulty,
the fields of the rspec are meaningful only to the individual
schedulers on each node.
Second, there is significant interest in developing marketbased brokerage services that establish resource value based
on demand, and a site’s purchasing capacity based on the resources it contributes. PLC currently has a simple model in
which each site receives an equal number of shares that it redistributes to projects at that site. PLC also allocates shares
directly to certain infrastructure services, including experimental brokerage services that are allowed to redistribute
them to other services. In the long term, we envision each
site administrator deciding to employ different, or possibly
even multiple, brokerage services, giving each some fraction
of its total capacity. (In effect, site administrators currently
allocate 100% of their resources to PLC by default.)
To date, PLC supports two additional brokerage services:
Emulab and SHARP. Emulab supports short-lasting network
experiments by pooling a set of PlanetLab resources and establishing a batch queue that slices are able to use to serialize their access. This is useful during times of heavy demand, such as before conference deadlines. SHARP [17] is
a secure distributed resource management framework that allows agents, acting on behalf of sites, to exchange computational resources in a secure, fully decentralized fashion. In
SHARP, agents peer to trade resources with peering partners
using cryptographically signed statements about resources.
4.3
Network Virtualization
The PlanetLab OS supports network virtualization by providing a “safe” version of Linux raw sockets that services
can use to send and receive IP packets without root privileges. These sockets are safe in two respects. First, each raw
socket is bound to a particular TCP or UDP port and receives
traffic only on that port; conflicts are avoided by ensuring
that only one socket of any type (i.e., standard TCP/UDP or
raw) is sending on a particular port. Second, outgoing packets are filtered to make sure that the local addresses in the
headers match the binding of the socket. Safe raw sockets
support network measurement experiments and protocol development on PlanetLab.
Safe raw sockets can also be used to monitor traffic within
a slice. A “sniffer” socket can be bound to any port that is
already opened by the same VM, and this socket receives
copies of all packet headers sent and received on that port.
Additionally, sufficiently privileged slices can open a special
administrative sniffer socket that receives copies of all outgoing packets on the machine tagged with the context ID of the
sending vserver; this administrative socket is used to implement the traffic monitoring facility described in Section 4.4.
4.3.1
Interface
A standard Linux raw socket captures all incoming IP packets and allows writing of arbitrary packets to the network. In
contrast, a safe raw socket is bound to a specific UDP or TCP
port and receives only packets matching the protocol and port
to which it is bound. Outgoing packets are filtered to ensure
that they are well-formed and that source IP and UDP/TCP
port numbers are not spoofed.
Safe raw sockets use the standard Linux socket API with
minor semantic differences. Just as in standard Linux,
first a raw socket must be created with the socket system call, with the difference that it is necessary to specify
IPPROTO TCP or IPPROTO UDP in the protocol field. It
must then be bound to a particular local port of the specified
protocol using the Linux bind system call. At this point the
socket can send and receive data using the usual sendto,
sendmsg, recvfrom, recvmsg, and select calls. The
data received includes the IP and TCP/UDP headers, but not
the link layer header. The data sent, by default, does not need
to include the IP header; a slice that wants to include the IP
header sets the IP HDRINCL socket option on the socket.
ICMP packets can also be sent and received through safe
raw sockets. Each safe raw ICMP socket is bound to either a local TCP/UDP port or an ICMP identifier, depending on the type of ICMP messages the socket will receive
and send. To get ICMP packets associated with a specific
local TCP/UDP port (e.g., Destination Unreachable, Source
Quench, Redirect, Time Exceeded, Parameter Problem), the
ICMP socket needs to be bound to the specific port. To exchange ICMP messages that are not associated with a specific
TCP/UDP port—e.g., Echo, Echo Reply, Timestamp, Timestamp Reply, Information Request, and Information Reply—
the socket has to be bound to a specific ICMP identifier (a
16-bit field present in the ICMP header). Only messages containing the bound identifier can be received and sent through
a safe raw ICMP socket.
PlanetLab users can debug protocol implementations or
applications using “sniffer” raw sockets. Most slices lack the
necessary capability to put the network card into promiscuous mode and so cannot run tcpdump in the standard way.
A sniffer raw socket can be bound to a TCP or UDP port
that was previously opened in the same vserver; the socket
receives copies of all packets sent or received on the port but
cannot send packets. A utility called plabdump opens a
sniffer socket and pipes the packets to tcpdump for parsing, so that a user can get full tcpdump-style output for any
of his or her connections.
4.3.2
Implementation
Safe raw sockets are implemented by the SILK kernel module, which intercepts all incoming IP packets using Linux’s
netfilter interface and demultiplexes each to a Linux
socket or to a safe raw socket. Those packets that demultiplex to a Linux socket are returned to Linux’s protocol stack
for further processing; those that demultiplex to a safe raw
socket are placed directly in the per-socket queue maintained
by SILK. When a packet is sent on a safe raw socket, SILK
intercepts it by wrapping the socket’s sendmsg function
in the kernel and verifies that the addresses, protocol, and
port numbers in the packet headers are correct. If the packet
passes these checks, it is handed off to the Linux protocol
stack via the standard raw socket sendmsg routine.
SILK’s port manager maintains a mapping of port assignments to vservers that serves three purposes. First, it ensures that the same port is not opened simultaneously by a
TCP/UDP socket and a safe raw socket (sniffer sockets excluded). To implement this, SILK must wrap the bind,
connect, and sendmsg functions of standard TCP/UDP
sockets in the kernel, so that an error can be returned if an
attempt is made to bind to a local TCP or UDP port already
in use by a safe raw socket. In other words, SILK’s port
manager must approve or deny all requests to bind to a port,
not just those of safe raw sockets. Second, when bind is
called on a sniffer socket, the port manager can verify that
the port is either free or already opened by the vserver attempting the bind. If the port was free, then after the sniffer
socket is bound to it the port is owned by that vserver and
only that vserver can open a socket on that port. Third, SILK
allows the node manager described in Section 4.2.1 to reserve specific ports for the use of a particular vserver. The
port manager stores a mapping for the reserved port so that
it is considered owned by that vserver, and all attempts by
other vservers to bind to that port will fail.
4.3.3
Discussion
The driving application for safe raw sockets has been the
Scriptroute [36] network measurement service. Scriptroute
provides users with the ability to execute measurement
scripts that send arbitrary IP packets, and was originally written to use privileged raw sockets. For example, Scriptroute
implements its own versions of ping and traceroute,
and so needs to send ICMP packets and UDP packets with the
IP TTL field set. Scriptroute also requires the ability to generate TCP SYN packets containing data to perform sprobestyle bottleneck bandwidth estimation. Safe raw sockets allowed Scriptroute to be quickly ported to PlanetLab by simply adding a few calls to bind. Other users of safe raw sockets are the modified versions of traceroute and ping
that run in a vserver (on Linux, these utilities typically run
with root privileges in order to open a raw socket). Safe raw
sockets have also been used to implement user-level protocol stacks, such as variants of TCP tuned for high-bandwidth
pipes [20], or packet re-ordering when striping across multiple overlay paths [43]. A BSD-based TCP library currently
runs on PlanetLab.
Safe raw sockets are just one example of how PlanetLab
services need to be able to share certain address spaces. Another emerging example is that some slices want to customize
the routing table so as to control IP tunneling for their own
packets. Yet another example is the need to share access to
well-known ports; e.g., multiple services want to run DNS
servers. In the first case, we are adopting an approach similar
to that used for raw sockets: partition the address space by
doing early demultiplexing at a low level in the kernel. In
the second case, we plan to implement a user-level demultiplexor. In neither case is a single service granted privileged
and exclusive access to the address space.
4.4
Monitoring
Good monitoring tools are clearly required to support a distributed infrastructure such as PlanetLab, which runs on hundreds of machines worldwide and hosts dozens of network
services that use and interact with each other and the Internet in complex and unpredictable ways. Managing this
infrastructure—collecting, storing, propagating, aggregating,
discovering, and reacting to observations about the system’s
current conditions—is one of the most difficult challenges
facing PlanetLab.
Consistent with the principle of unbundled management,
we have defined a low-level sensor interface for uniformly
exporting data from the underlying OS and network, as well
as from individual services. Data exported from a sensor can
be as simple as the process load average on a node or as complex as a peering map of autonomous systems obtained from
the local BGP tables. That is, sensors encapsulate raw observations that already exist in many different forms, and pro-
vide a shared interface to alternative monitoring services.
4.4.1
Interface
A sensor provides pieces of information that are available (or
can be derived) on a local node. A sensor server aggregates
several sensors at a single access point, thereby providing
controlled sharing of sensors among many clients (e.g., monitoring services). To obtain a sensor reading, a client makes
a request to a sensor server. Each sensor outputs one or more
tuples of untyped data values. Every tuple from a sensor conforms to the same schema. Thus, a sensor can be thought of
as providing access to a (potentially infinite) database table.
Sensor semantics are divided into two types: snapshot and
streaming. Snapshot sensors maintain a finite size table of
tuples, and immediately return the table (or some subset of
it) when queried. This can range from a single tuple which
rarely varies (e.g. “number of processors on this machine”)
to a circular buffer that is constantly updated, of which a
snapshot is available to clients (for instance, “the times of
100 most recent connect system calls, together with the associated slices”). Streaming sensors follow an event model,
and deliver their data asynchronously, a tuple at a time, as it
becomes available. A client connects to a streaming sensor
and receives tuples until either it or the sensor server closes
the connection.
More precisely, a sensor server is an HTTP [15] compliant
server implementing a subset of the specification (GET and
HEAD methods only) listening to requests from localhost
on a particular port. Requests come in the form of uniform
resource identifiers (URIs) in GET methods. For example,
the URI:
http://localhost:33080/nodes/ip/name
is a request to the sensor named “nodes” at the sensor server
listening on port 33080. The portion of the URI after the
sensor name (i.e., ip/name) is interpreted by the sensor. In
this case, the nodes sensor returns comma-separated lines
containing the IP address and DNS name of each registered
PlanetLab node. We selected HTTP as the sensor server protocol because it is a straightforward and well-supported protocol. The primary format for the data returned by the sensor
is a text file containing easy-to-parse comma separated values.
4.4.2
Implementation
An assortment of sensor servers have been implemented to
date, all of which consist of a stripped-down HTTP server encapsulating an existing source of information. For example,
one sensor server reports various information about kernel
activities. The sensors exported by this server are essentially
wrappers around the /proc file system. Example sensors
include meminfo (returns information about current memory usage), load (returns 1-minute load average), load5
(returns 5-minute load average), uptime (returns uptime of
the node in seconds), and bandwidth(slice) (returns
the bandwidth consumed by a slice, given by a slice id).
These examples are simple in at least two respects. First,
they require virtually no processing; they simply parse and
filter values already available in /proc. Second, they neither stream information nor do they maintain any history.
One could easily imagine a variant of bandwidth, for example, that both streams the bandwidth consumed by the
slice over that last 5 minute period, updated once every five
minutes, or returns a table of the last n readings it had made.
In contrast, a more complex sensor server will shortly become available that reports information about how the local
host is connected to the Internet, including path information
returned by traceroute, peering relationships determined
by a local BGP feed, and latency information returned by
ping. This sensor server illustrates that some sensors may
be expensive to invoke, possibly sending and receiving messages over the Internet before they can respond, and as a result may cache the results of earlier invocations.
4.4.3
Discussion
Using the sensor abstraction, and an emerging collection of
sensor implementations, an assortment of monitoring services are being deployed. Many of these services are modeled as distributed query processors, including PIER [19],
Sophia [40], and IrisNet [29].
The long-term goal is for these monitoring services to detect, reason about, and react to anomalous behavior before
it becomes disruptive. However, PlanetLab has an immediate need of responding to disruptions after the fact. Frequently within the past year, traffic generated by PlanetLab
researchers has caught the attention of ISPs, academic institutions, Web sites, and sometimes even home users. In nearly
all cases, the problems have stemmed from naı̈ve service design and analysis, programmer errors, or hyper-sensitive intrusion detection systems. Examples include network mapping experiments that probe large numbers of random IP addresses (looks like a scanning worm), services aggressively
traceroute’ing to certain target sites on different ports
(looks like a portscan), services performing distributed measurement to a target site (looks like a DDoS attack), services
sending large numbers of ICMP packets (not a bandwidth
problem, but renders low-end routers unusable), and so on.
Addressing such complaints requires an auditing tool that
can map an incident onto a responsible party.
Specifically, a traffic auditing service runs on every PlanetLab node, snooping all outgoing traffic using the administrative raw sniffer socket provided by the SILK module that
tags each packet with the ID of the sending vserver. From
each packet, the auditing service logs the time it was sent,
the IP source and destination, protocol, port numbers, and
TCP flags if applicable. It then generates Web pages on each
node that summarize the traffic sent in the last hour by IP
destination and slice name. The hope is that an administrator
at a site that receives questionable packets from a PlanetLab
machine will type the machine’s name or IP address into his
or her browser, find the audit-generated pages, and use them
to contact the experimenters about the traffic. For example,
an admin who clicks on an IP address in the destination summary page is shown all of the PlanetLab accounts that sent a
packet to that address within the last hour, and provided with
links to send email to the researchers associated with these
accounts. Another link directs the admin to the network traffic database at www.planet-lab.org where back logs are
archived, so that he or she can make queries about the origin
of traffic sent earlier than one hour ago.
Our experience with the traffic auditing service has been
mixed. On the one hand, the PlanetLab support team has
found it very useful for responding to traffic queries: after
receiving a complaint, they use the Web interface to identify the responsible party and forward the complaint on to
him. As a result, there has been a reduction in overall incident response time and the time invested by support staff
per incident. On the other hand, many external site administrators either do not find the web page or choose not to
use it. For example, when receiving a strange packet from
planetlab-1.cs.princeton.edu, most sites respond by
sending email to [email protected]; by the time the
support team receives the complaint, it has bounced through
several levels of university administration. We may be able
to avoid this indirection by providing reverse DNS mappings
for all nodes to nodename.planet-lab.org, but this requires effort from each site that sponsors PlanetLab nodes.
Finding mechanisms that further decentralize the problemresponse process is ongoing work.
Finally, although our experience to date has involved implementing and querying read-only sensors that can be safely
accessed by untrusted monitoring services, one could easily imagine PlanetLab also supporting a set of actuators
that only trusted management services could use to control
PlanetLab. For example, there might be an actuator that terminates a slice, so that a Sophia expression can be written to
kill a slice that has violated global bandwidth consumption
limits. Today, slice termination is not exposed as an actuator,
but is implemented in the node manager; it can be invoked
only by the trusted PLC service, or an authenticated network
operator that remotely logs into the node manager.
5
Evaluation
This section evaluates three aspects of slice creation and initialization.
5.1
Vserver Scalability
The scalability of vservers is primarily determined by disk
space for vserver root filesystems and service-specific storage. On PlanetLab, each vserver is created with a root
filesystem that points back to a trimmed-down reference root
filesystem which comprises 1408 directories and 28003 files
covering 508 MB of disk. Using vserver’s primitive COW
on all files, excluding those in /etc and /var, each vserver
root filesystem mirrors the reference root filesystem while
only requiring 29 MB of disk space, 5.7% of the original root
filesystem size. This 29 MB consists of 17.5 MB for a copy
of /var, 5.6 MB for a copy of /etc, and 5.9 MB to create
1408 directories (4 KB per directory). Given the reduction
in vserver disk footprints afforded by COW, we have been
able to create 1,000 vservers on a single PlanetLab node. In
the future, we would like to push disk space sharing even further by using a true filesystem COW and applying techniques
from systems such as the Windows Single Instance Store [7].
Kernel resource limits are a secondary factor in the scalability of vservers. While each vserver is provided with the
illusion of its virtual execution environment, there still remains a single copy of the underlying operating system and
associated kernel resources. Under heavy degrees of concurrent vserver activity, it is possible that limits on kernel resources may become exposed and consequently limit system
scalability. (We have already observed this with file descriptors.) The nature of such limits, however, are no different
from that of large degrees of concurrency or resource usage
within a single vserver or even on an unmodified Linux kernel. In both cases, one solution is to simply extend kernel
resource limits by recompiling the kernel. Of course, simple
scaling up of kernel resources may be insufficient if inefficient algorithms are employed within the kernel (e.g., O(n)
searches on linked lists). Thus far, we have yet to run into
these types of algorithmic bottlenecks.
5.2
Slice Creation
This section reports how long it takes the node manager to
create a vserver on a single node. The current implementation of PLC has each node poll for slice creation instructions
every 10 minutes, but this is an artifact of piggybacking the
slice creation mechanism on existing software update machinery. It remains to be seen how rapidly a slice can be
deployed on a large number of nodes.
To create a new slice on a specific node, a slice creation
service must complete the following steps at that node:
1. the slice creation service contacts a port mapping service to find the port where the node manager’s XMLRPC server is listening for requests;
2. the slice creation service performs a node manager acquire RPC to obtain an rcap for immediate rights to a
vserver and best-effort resource usage;
3. the slice creation service performs a node manager bind
RPC to bind the ticket to a new slice name;
4. the node manager, after completing the RPCs, creates
the new vserver and notifies the necessary resource
schedulers to effect the newly added resource bindings
for the new slice; and
5. the node manager calls vadduser to instantiate the
vserver and then calls vserver-init to start execution of
software within the new vserver.
Running on a 1.2GHz Pentium, the first three steps complete in 0.15 seconds, on average. How long the fourth and
fifth steps takes depends on how the user wants to initialize
the slice. At a minimum, the vserver creation and initialization takes an additional 9.66 seconds on average. However,
this does not include the time to load and initialize any service software such as sshd or other packages. It also assumes a hit in a warm cache of vservers. Creating a new
vserver from scratch takes over a minute.
5.3
Service Initialization
This section uses an example service, Sophia [40], to demonstrate how long it takes to initialize a service once a slice exists. Sophia’s slice is managed by a combination of RPM,
apt-get, and custom slice tools. When a Sophia slice
is created, it must be loaded with the appropriate environment. This is accomplished by executing a boot script inside each vserver. This script downloads and installs the
apt-get tools and a root Sophia slice RPM, and starts an
update process. Using apt-get, the update process periodically downloads the tree of current packages specific for
the Sophia slice. If a newer package based on the RPM hierarchy is found, it and its dependencies are download and installed. With this mechanism, the new versions of packages
are not directly pushed to all the nodes, but are published
in the Sophia packages tree. The slice’s update mechanism
then polls (potentially followed with an action request push)
the package tree and performs the upgrade actions.
In the current setting, it takes on average 11.2 seconds to
perform an empty update on a node; i.e., to download the
package tree, but not find anything new to upgrade. When
a new Sophia “core” package is found and needs to be upgraded, the time increases to 25.9 seconds per node. These
operations occur in parallel, so the slice upgrade time is not
bound by the sum of node update times. However, the slice
is to be considered upgraded only when all of its active nodes
are finished upgrading. When run on 180 nodes, the average
update time (corresponding to the slowest node) is 228.0 seconds. The performance could be much improved, for example, by using a better distribution mechanism. Also, a faster
alternative to the RPM package dependencies system could
improve the locally performed dependency checks.
6
Conclusions
Based on experience providing the network research community with a platform for planetary-scale services, the PlanetLab OS has evolved a set of mechanisms to support distributed virtualization and unbundled management. The design allows network services to run in a slice of PlanetLab’s
global resources, with the PlanetLab OS providing only local
(per-node) abstractions and as much global (network-wide)
functionality as possible pushed onto infrastructure services
running in their own slices. Only slice creation (coupled
with resource allocation) and slice termination run as a global
privileged service, but in the long-term, we expect a set of alternative infrastructure services to emerge and supplant these
bootstrap services.
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