DIMMer: A case of turning off DIMMs in clouds.

DIMMer: A case of turning off DIMMs in clouds.
Dongli Zhang, Moussa Ehsan, Michael Ferdman, Radu Sion
Stony Brook University
{dozhang, mehsan, mferdman, sion}@cs.stonybrook.edu
1. Introduction
Lack of energy proportionality in server systems results in
significant waste of energy when operating at low utilization,
a common scenario in today’s data centers. However, even
during times of low utilization, servers cannot be powered
off because of unpredictable spikes in instantaneous demand
on some of the resources (e.g., file storage). To mitigate energy waste, unused processor cores transition into low-power
sleep states, power-gating non-critical components; memory
ranks (physical subdivisions of capacity) transition into selfrefresh modes, where energy is spent only to maintain memory contents.
We propose DIMMer, an approach to entirely eliminate
the idle power consumption of unused system components.
DIMMer is motivated by two key observations. First, that
even in their lowest-power states, the power consumption
of today’s server components remains significant. Second,
that unused components can be powered off entirely without
sacrificing availability. We demonstrate that unused memory capacity can be turned off, eliminating the energy waste
of self-refresh for unallocated memory, while still allowing
for all capacity to be available on a moment’s notice. Similarly, only one CPU socket must remain powered on, allowing unused CPUs and attached memory to be powered off
entirely. Leaving a single CPU powered on allows the server
to remain fully operational and capable of rapidly ramping
up to peak capacity if needed. In this work, we demonstrate
the potential for DIMMer to improve energy proportionality
and achieve energy savings. Using power measurements of
a modern server system and data from a Google data center,
we show up to 50% savings on DRAM and 18.8% on CPU
background energy.
To handle hundreds of millions of users and their associated transactions, companies such as Amazon, Facebook,
and Google run immense data centers with until-recently
unimaginable computation and storage capacities. As online services become pervasive, projections indicate that
electricity consumed in global data centers worldwide in
2010 is more than 200B KWh, between 1.1% and 1.5%
of worldwide electricity use [20]. Three years ago, Google
announced that their facilities have a continuous electricity
usage equivalent to powering 200,000 homes [11].
Surprisingly, despite energy being one of the top three
data center operating costs [13], much of the data center energy is wasted because data centers cannot modulate capacity according to demand. Even when experiencing frequent
periods of complete inactivity (idle periods upwards of one
second [25] during times of low utilization), servers are kept
operating at full capacity. Across data centers, hundreds of
thousands of servers remain idle or underutilized in anticipation of spontaneous demand spikes [6]. As a result, a report
by New York Times found energy waste upwards of 90% as
the facilities are operated at full capacity regardless of the
demand [12].
Industry has a number of energy saving principles and
mechanisms, such as consolidation, virtualization (for increased utilization), decommissioning of unused servers,
and the purchase of energy-efficient hardware [3]. Such
mechanisms show promise and research demonstrates that
job consolidation and server power-off strategies can result
in up to 50% savings [34]. Nevertheless, despite their theoretical promise, these techniques are rarely used due to the
need for fast response times to instantaneous demand and
the increased failure rates of mechanical components such
as hard disks and fans due to frequent power cycling [27].
Motivated by the fact that complete server power-off
strategies are not appropriate in many data centers, we propose an alternative that can modulate energy use based on
capacity demand by turning off independent hardware components. In this paper, we envision DIMMer, a system to
provide an agile framework for workload-driven scalability
and power reduction in data centers. DIMMer turns off all
idle DRAM ranks (physical subdivisions of memory capacity) in data center servers during low resource utilization to
Acknowledgement: This research was supported in part by NSF grants NSF CNS1161541, NSF CNS-1318572, NSF CNS-1223239, NSF CCF-0937833, by US ARMY
award W911NF-13-1-0142, and by gifts from Northrop Grumman Corporation,
Parc/Xerox, and Microsoft Research.
Submission to SoCC ’14, October, 2014, Seattle, WA, USA.
save DRAM background power. Prior work either applies
dynamic voltage and frequency scaling (DVFS) to DRAM
[9, 10], or maximizes the time that DRAMs spend in lowpower modes [17, 22, 33] (i.e., self-refresh mode). We observe that dynamic control of memory capacity presents
an opportunity to reduce energy consumption and promote
power proportionality of server systems. While prior work
has reduced the time that DRAMs spend in high-power
modes, we find that reducing the memory space available to
the system can yield even greater benefits: energy costs for
additional DRAM capacity are paid only when this capacity
is requested by the system. Current systems waste this energy, because even when the DRAM is in self-refresh mode,
the power consumption of a 4GB DIMM is approximately
1W (20% of the precharge-standby power [9]). Furthermore,
because self-refresh power is proportional to DRAM capacity, the savings of DIMMer are likely to be even higher in
future systems. In this paper, we use publicly available traces
from production Google data centers to demonstrate the effectiveness of DIMMer.
A further benefit of DIMMer over switching DRAM to
self-refresh mode [33] is that freeing DIMM contents and
turning them off permits also powering down unused CPUs
to which these DIMMs are attached, whereas using selfrefresh modes and retaining memory contents forces keeping
the CPUs powered up, even if all cores in those CPUs are unused. Although DIMMer requires disabling DRAM channel
interleaving, migrating memory pages among DRAM ranks,
and reducing the memory available for disk cache, we find
that these requirements do not impact the effectiveness of
DIMMer. Moreover, while DIMMer requires modification
to the OS kernel and hardware, there are no modifications to
the applications [7, 23]. Finally, many of DIMMer’s prerequisites are already implemented in prior work [7, 19, 23, 33].
Applying DIMMer to Google cluster traces [28] demonstrates that background DRAM and CPU energy consumption can be reduced by up to 50% and 18.8%, respectively.
Figure 1. 8GB server DIMM power consumption as a function of throughput. Power at near-zero throughput is more
than half of the power at peak utilization.
2.2 CPU Power Consumption
Our experience shows that individual core power consumption varies significantly across different CPUs even within
the same server and is difficult to measure precisely and
consistently (it varies with the number of per-node DIMMs,
channels, utilization, etc.). For the purpose of this evaluation,
we chose to be conservative and consider power consumption measured across CPUs rather than individual cores. The
measured power consumptions of our test system while completely idle are as follows: for 1 CPU (Intel(R) Xeon(R) CPU
E5-2650 2.00GHz) and 4 DIMMs: 32W; for 2 CPUs and 4
DIMMs: 48W; and for 2 CPUs and 8 DIMMs: 52W. We find
that keeping an idle CPU powered up only to maintain contents of the 4 DIMMs attached to it consumes 16W, indicating significant opportunity to save energy by powering down
the CPU in addition to memory at times of low utilization.
3. DIMMer Benefits
In an ideal world, servers can be turned on or off at zero latency and cost. In that world, one would strive to do exactly
that; components would be powered off as soon as a server’s
utilization can be reduced to zero through migrating applications and consolidation.
The real world however, imposes a set of rigid latencyrelated constraints on this vision that cannot be ignored. Migrating jobs and turning servers off and back on are high latency and high cost operations. Additionally, often (as in the
case of the Google dataset [28] discussed below) individual
servers may participate in distributed services (e.g., file serving) that preclude turning off machines because they must be
able to serve file contents on a moment’s notice. As a result,
clouds end up operating hundreds of thousands of servers at
full capacity even at periods of low load, in anticipation of
spontaneous demand spikes [6].
In this paper, we show that DIMMer, which dynamically
provisions memory capacity at the granularity of DRAM
ranks, represents an opportunity to reduce energy consumption of server systems while having little to no impact on
performance. To demonstrate that DIMMer is a viable design in practical settings, we analyze the main trade-offs and
2. Motivation for Powering off Components
2.1 DRAM Power Consumption
Figure 1 depicts our measurements of typical power consumption of active DIMMs under nominal use in a server
rack. Measured DIMMs (Samsung 1600MHz Dual-Rank
ECC) were physically isolated in a dedicated socket – the illustrated data is for one of the 8GB DIMMs, installed alone
at the second CPU of a PowerEdge M620 server. Power
consumption was measured with increasing throughput up
to 2GB/s, a reasonable upper bound for modern data center
workloads [24]. We find that simply keeping a DIMM powered on with near-zero memory traffic has a constant power
consumption of 2.3W, which constitutes more than 50% of
each DIMM’s power consumption observed at peak throughput for cloud workloads.
(a) Idle server memory, measured at rank granularity
(b) Idle CPUs, measured at CPU socket granularity
Figure 2. Total number of idle DRAM ranks and CPUs cluster-wide in each 5-minute time slot. DRAM capacities, rank, CPU,
and server counts are shown in Table 1.
show that: (i) idle power consumption of DRAM ranks is
high and warrants full power-off, (ii) existing clouds feature
numerous- and long-enough (per-server, per-rank) idle periods to justify the DIMMer overheads and latencies, (iii) the
resulting cloud power savings are significant.
Memory Capacity
Total Machines:
3.1 Idle Resources in the Cloud
To justify the overheads and latencies incurred by DIMMer,
we must establish that real clouds feature numerous- and
long-enough (per-server, per-rank) idle periods.
Google Cluster Traces. We analyzed a cluster usage dataset
released by Google [28]. The dataset consists of workload
traces for over 12,000 servers collected at 5-minute granularity over the course of more than one month. The traces
include detailed information about the servers and the workload jobs and tasks, including CPU, memory, and storage
per task, and machine resource utilization. This dataset has
spawned a number of seminal results [2].
We first compute the DRAM and CPU utilization of each
server in every 5-minute interval. Based on this, we derive
the total cluster-wide number of idle memory ranks and
CPUs for each 5-minute time slot. Figure 2(a) illustrates the
idle DRAM results. We find that up to 50% of the cluster
DRAM ranks are unused and can be powered off.
Figure 2(b) illustrates the number of CPUs that can be
turned off in each 5-minute time slot. Unlike DRAM ranks
that can be turned off independently, a CPU can be powered
off only when all of its cores and all DRAM ranks attached
to its socket are idle. In the existing trace, on average, 20% of
the CPUs can be powered off across Google’s cluster, while
the aggregate CPU utilization is 50%. Changes to the cluster
resource management framework can mitigate this inconsistency and maximize the number CPUs that can be powered
off. However, for the remainder of this study, we use the exact data available in Google traces, showing DIMMer’s effectiveness with the existing job placement policies.
Figure 3 shows that DIMMer can save 30MWh (DRAM)
and 52MWh (CPU) for this cluster running for one month.1
Using the cost model from [14], we estimate the corresponding cost saving over the Total Cost of Ownership (TCO),
1 Using
in Dataset
Table 1. The dataset used for our study provides relative
memory capacities normalized to the maximum-capacity
machine present in the cluster. We estimate per-machine
DRAM capacity and number of DIMMs based on the distribution of the machine counts in the dataset, the approximate date of cluster deployment, and the fact that Google
populated all server DRAM slots at that time [29]. Machines
with unusual memory capacities (likely due to partial memory failures) were ignored.
including data center construction, IT equipment, and operating cost at 0.6% (over total cost), 1.4% (over total power
cost) and 3.1% (over total power cost, excluding power for
cooling), respectively.2 These energy savings also translate
to a significant reduction in environmental pollution. According to the EPA Emissions & Generation Resource Integrated Database (eGRID) [1], this corresponds to a U.S.
annual non-baseload CO2 output emission reduction of over
51 metric tons of CO2 .
3.2 Power-off vs. Self-refresh
Prior work proposed to maximize the time DRAM ranks
spend in low-power self-refresh mode [17, 22, 33]. Although
these techniques effectively reduce DRAM power consumption, as shown in section 2, the background power of an 8GB
DIMM in self-refresh mode is actually higher than the savings achieved by self-refresh compared to the peak power
consumption of DRAM for a typical cloud workload.
Table 2 shows that DIMMer can significantly reduce
wasted power by turning off idle memory and CPUs. For
2 We assume $0.1/kWh, $50M facility cost, and 1.2 PUE for 12,583 servers.
Facility and IT capital costs are amortized over 15 and 3 years, respectively.
DRAM power estimates from [9].
Self-Refresh [33]
(2 idle ranks)
(2 idle ranks)
Percentage of Time in ACT STBY/PRE STBY
Rank 0/1 Rank 2/3 Rank 4/5 Rank 6/7
Node 1
Node 2
Node 1
Node 2
Self Refresh
Table 2. Sample system with 2 CPU sockets, each having two channels with 8 ranks. In an ideal case, the system would
consume 66W when consolidating hot memory pages on “hot” ranks and switching the “cold” ranks to self-refresh mode
(using [33]’s approach). Using DIMMer, if we instead turn off the “self-refresh”ed ranks we can save an additional 3% of the
background power consumption.
Percentage of Time in ACT STBY/PRE STBY
Self Refresh STBY
Rank 0/1 Rank 2/3 Rank 4/5 Rank 6/7
Self-Refresh [33] Node 1
Node 2
(1 idle node)
Node 1
Node 2
(1 idle node)
Table 3. The main opportunity arises when turning off all of the ranks – this corresponds to 20% memory nodes in Google
cluster as in Figure 2(b). If all hot memory pages are migrated to the “hot” memory node and the “cold” node’s ranks are placed
in self-refresh mode, 57.12W is consumed. If “cold” ranks are turned off completely, the entire CPU socket can be turned off,
resulting in an additional saving of 35% of the total background power consumption.
4. Vision for Implementation
This paper presents DIMMer as a vision. However, actual
implementation is not complex. DIMMer requires modifications to the memory management subsystem of the OS
kernel. Unlike the traditional Linux kernel which maintains
a memory free list for each memory zone (ZONE DMA,
ZONE DMA32 and ZONE NORMAL), DIMMer’s allocator creates a free list for each DRAM rank. Similar functionality has already been implemented in [7, 19]. The difference
in page allocation between DIMMer and standard Linux lies
in the total number of free lists. Besides an allocator, a page
migrator running as a kernel thread would be responsible
for on-demand memory page migration, moving cold pages
from “cold” ranks. Unlike prior work [7, 23], no libraries or
application would need to be modified.
For reliable deployment, DIMMer may also requires
hardware changes. Flikker [23] has already proposed to reduce the memory refresh power consumption by decreasing
DRAM refresh rate. Theoretically, we can remove most selfrefresh power by setting refresh rates to zero. However, to
reduce the total self-refresh power to zero, we hope hardware
manufacturers will expose registers that allow full electrical
power-off for entire DRAM ranks in the next generation
DRAM controllers.3 Support for full electrical power-off of
CPU sockets may also be helpful.
Figure 3. Energy saved by turning off the idle DRAM ranks
and CPUs, respectively.
example, a dual-socket server where each CPU is connected
to eight memory ranks (four dual-rank DIMMs) across two
memory channels, will consume 66W when using techniques that arrange memory into “cold” and “hot” ranks, saving energy on “cold” ranks by putting them into self-refresh
mode [33]. If we turn off “cold” ranks entirely, DIMMer can
save additionally 3% of the DRAM power consumption.
Furthermore, the main opportunity for power savings
arises when all of the ranks attached to a CPU can be turned
off (e.g., as would be the case in the Google cluster, where
DRAM utilization of many servers often falls below 50%).
In this case, DIMMer can further reduce background power
consumption by turning off idle memory ranks and their
corresponding CPUs. As shown in Table 3, this results in
an additional 35% reduction in the background power consumption when compared to the self-refresh approach.
3 We
suspect the functionality already exists in modern DRAM controllers,
but the control registers to power off ranks are not publicly documented.
5. DIMMer Costs
an active working set, disk caches do not noticeably affect
system performance.
Experimental evidence with disk caches in cloud workloads support this estimate. Zhu et al. indicate that, for a web
server, a large decrease in cache capacity leads to minimal
changes in hit rate [36]. Sacrificing a few percent hit rate,
the cache costs can be reduced by almost 90%, without any
noticeable effects on users’ experience.
Furthermore, prior work offers mechanisms to mitigate
cache capacity concerns [35]. Cache entries can be dynamically classified as “hot” or “cold”, and kept in separate ranks.
As certain cache pages become hotter, and their “cold” rank
host becomes a candidate to transition to low-power state,
the hot cache pages can be migrated to a “hot” rank.
Finally, note that any and all performance impact of DIMMer mechanisms can be disabled on demand at high utilization and only employed when the load (memory and CPU)
warrant their use.
5.1 Cost of Page Migration
Before powering off unused ranks, DIMMer migrates memory pages (generally of 4KB sizes) for consolidation onto the
active ranks. To estimate the energy cost of page migration,
we measured the energy consumption of migrating 8GB of
memory (4 DRAM ranks) from one NUMA node to another.
The node-to-node migration is the most expensive migration
that would happen in DIMMer.
The average measured energy cost to migrate a single
page is 102 µJ. As Table 4 shows, if DIMMer migrates 8GB
every 30 minutes, the additional monthly energy cost of page
migration for the cluster is approximately 210KWh, a mere
0.26% of DIMMer’s total savings.
Migration Frequency
Every 5-min
Every 15-min
Every 30-min
Every 1-hour
Percentage over total saving
5.3 Cost of Non-Interleaved Address Mapping
Table 4. Measured energy consumption of page migration,
expressed as absolute energy and as the percentage of DIMMer’s energy savings.
Rank-aware memory allocation can be achieved by disabling
both rank and channel interleaving [4], which may degrade
performance for some workloads. Channel interleaving is
used to improve memory bandwidth by interleaving physical
pages across DIMMs on multiple channels. Rank interleaving reduces memory latency by spreading each page across
many ranks, enabling concurrent accesses by letting the controller open a row in one rank, while another rank is being
accessed. However, for cloud workloads, the performance
reduction from disabling interleaving is negligible.
The main insight comes from the fact that most cloud
workloads severely underutilize the available memory bandwidth [24], even during peak times. Ferdman et al. show that
the per-core off-chip bandwidth utilization of MapReduce,
media streaming, web front end, and web search is at most
25% of the available bandwidth. As a result, the peak bandwidth reduction associated with disabling channel interleaving will not impact the performance of cloud applications.
Similar to turning off channel interleaving, turning off
rank interleaving will not incur an obvious performance reduction. For the niche of memory intensive workloads that
may get impacted, VipZonE [7] shows that turning off rank
interleaving results in only 1.03% execution time overhead.
Further, DIMMer can be designed to reserve certain
interleaving-enabled DRAM channels (e.g., on CPU socket
0, which will always remain powered on) to service memoryintensive workloads. Finally, it is also possible to retain the
benefits of channel interleaving by only turning off parallel
ranks across channels.
We also measured the performance penalty of page migration. It takes at most 13.5s to migrate 8GB of memory
in our test system. Although the time is not trivial, it is an
upper-bound. Further, it is important to note that this penalty
occurs by design only at low CPU and memory utilization,
when DIMMer is engaged to power off components. This is
exactly the time when unused CPU and memory bandwidth
are available. During high utilization, DIMMer can be designed to simply disable its allocator and migration thread.
Further, based on Google cluster traces, 30.2% of used
pages are cache pages and 11.2% are cache pages not
mapped into any processes. In many workloads [36], very
few disk cache pages are hot; it is only useful for DIMMer
to migrate hot cache pages and anonymous pages. During
times of low utilization, DIMMer applies a smart page allocation policy that minimizes the total number of page migrations that will be needed before ranks can be powered down.
To avoid perturbing live services, DIMMer migrates pages
in the background and at low priority.
5.2 Cost of Reduced Cache Capacity
Modern OSes liberally use large amounts of idle memory
as disk cache, following the mantra “free memory is wasted
memory.” Turning off DRAM reduces the disk cache capacity and may impact performance and energy by forcing reread of disk contents.
To estimate the impact of reduced cache capacity, [18, 30,
31] suggest that cache miss rates follows “the 30% Rule”
(i.e., doubling the cache size decreases the miss rate by 30%
on average). Accordingly, hit cache benefits decrease with
larger cache size. Beyond a certain capacity that captures
6. Related Work
A number of works address energy proportionality at server
granularity. In [34], Zhang et al. dynamically change the
number of active machines in the cloud to save energy by
solving an optimization problem, finding the best tradeoff between the cost of reconfiguration and the amount of
energy consumed across the entire data center. Analyzing
Google cluster traces shows that this solution could have
saved 18.5% to 50% of the consumed energy. Krioukov et
al. [21] propose NapSAC, a power-proportional web cluster.
NapSAC provisions the number of machines needed for the
current workload and the observed response latency. Chen
et al. [8] also proposed an energy-aware server provisioning
approach for internet services by adaptively changing the
number of powered on servers. Unlike NapSAC, which concentrates on workloads with short-lived requests, Chen et
al. [8] handled workloads with long-lived connections (e.g.,
Windows Live Messenger). However, these approaches cannot be used if cloud servers provide background services and
cannot be powered off, providing an opportunity for DIMMer to achieve energy proportionality without losing availability. Moreover, instantaneous demand spikes and the increased failure rates of frequently power-cycled components
such as power supplies, disks, and fans further discourage
such power-off approaches [27].
Servers reduce power consumption during times of low
utilization by putting components into low-power states.
CPU cores and private caches use dynamic voltage and
frequency scaling (DVFS) to adaptively tune the CPU frequency to reduce its power consumption [15, 16]. When
completely inactive, CPU cores are power gated and memory DIMMs are placed into self-refresh states. DIMMer
takes this concept a step further, proposing to completely
power down entire CPUs when all cores and attached memories are unused.
Similar to the approaches that increase inactivity time
on disks, a number of proposals modify the OS page allocation policy and DRAM controller logic to maximize
the time that DRAM ranks spend in low-power states [5,
22]. Sparsh Mittal has surveyed several techniques that efficiently manage DRAM power consumption [26] (e.g., by reducing the power consumption of memory activation, memory read/write, transition among different power modes, and
by the utilization of low-power self-refresh mode). DIMMer extends these approaches to allow powering off unused
DRAM ranks.
Noting that data center workloads need high memory capacity, but under-utilize bandwidth, a number of approaches
improve memory energy proportionality by reducing performance. Using power-efficient mobile DRAM devices reduces server energy costs [24]. MemScale [10] proposes a
scheme to apply DVFS to the memory controller and dynamic frequency scaling (DFS) to the memory channels and
DRAM devices. Unlike our work, these approaches assume
that all memory capacity contains useful data, resulting in
an energy cost to refresh the entire memory capacity, even if
much of it is unused.
Due to the bursty nature of data center workloads, “PowerNap” proposes introducing low-power idle states into all
server components [25]. When work arrives, servers must
quickly transition to an operational state, perform work, and
return to the low-power idle mode. This work is complimentary to ours, as we propose identifying and powering off entirely unused cores and memory, further increasing the potential energy savings.
7. Conclusions
It is long-recognized that most server hardware exhibits disproportionately high energy consumption when operating at
low utilization [32]. To mitigate this effect, low-power operating modes have been introduced. Moreover, techniques
have been developed to maximize the time spent in lowpower states. DIMMer builds on this work and observes
that dynamic control of memory capacity presents an additional opportunity to reduce energy consumption of server
systems. Just as servers experience times of low CPU utilization, they also experience times of low memory capacity
demand. While prior work has concentrated on reducing the
time that components such as CPU and memory spend in
high-power modes, we find that reducing the memory space
available to the system can yield even greater benefits.
By dynamically reducing the memory capacity available
to the OS, we are able to consolidate unused memory on
DRAM ranks that can be entirely powered off rather than
simply being placed into a low-power state. Controlling capacity in this way enables true power proportionality for the
memory system, where the energy costs for memory capacity are paid only when this capacity is requested by the system. Moreover, turning off unused memory capacity enables
even greater energy savings on CPUs, as unused CPUs can
be powered off when all memory capacity attached to their
sockets is unused. DIMMer allows the server to remain fully
operational and capable of rapidly ramping up to peak capacity if needed, but saves significant energy by entirely powering off components rather than placing them into a lowpower state. Using publicly available Google cluster traces
and power measurements on a modern server system, we
demonstrated that applying the DIMMer approach can reduce DRAM and CPU background energy consumption up
to 50% and 18.8%, respectively. This corresponds to a U.S.
annual non-baseload CO2 output emission reduction of over
51 metric tons of CO2.
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