Software Strategies for Portable Computer Energy Management

IEEE Personal Communications 1998Presented by Hsu Hao Chen

OutlineIntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

Introduction(1/2)Limiting the energy consumption of computers, especially portables, is becoming increasingly important.

Goalreduce energy consumption is to simply use components that consume less power.use components that can enter low-power modes by temporarily reducing their speed or functionalitydiscuss software techniques for taking advantages of low-power hardware

Introduction(2/2)Power-saving three categories of software strategies

transition issueload-change issueadaptation issue

IntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

Strategy TypesTransition strategies (prediction strategies)

determining when to switch to low-power modesknowledge about its mode characteristicsinformation about its future functionality requirements

Load-change strategieshow to modify the load on a component in order to increase its use of low-power modes

Adaptation strategieshow to create software that allows components to be used in power-saving ways

Levels of Energy Management

Application Level

User Level

OS Level

Component Level

Abstraction

Lack informationabout the overall

workload

Lack certain useful information about the state of the machine

Application-aware adaptation

Strategy Evaluation and Power Budget

Strategy Evaluation

Power Budgetmechanical motion and light generation seem inherently more power intensive than digital circuits.

IntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

Hardware FeaturesMost hard disks have five power modes:

active

idle

standby

sleep

off

r/w controlspin cache

r/w controlspin cache

r/w controlspin cache

r/w controlspin cache

r/w controlspin cache

Transition StrategiesDisks can be of three kinds:

deciding when to go to sleep modedeciding when to go to standby mode (no studies)deciding when to go to off mode

Common strategyuse inactivity thresholds as low as 1-10 seconds (Researchers discovered )enter sleep mode after a fixed inactivity threshold (3-5min)

Problemsmake the user wait more often for the disk to spin-upcause disk failure from frequent spin up/down cyclesIt is very difficult to predict accurate disk access pattern due to wide-varying of human activities

Changing Inactivity ThresholdFirst argument

assumption: disk access interarrival times are drawn from some unknown stationary distributionkeep track of all interarrival times in order to deduce the best threshold

Second argumentassumption: disk access interarrival time distribution is nonstationary (i.e. changing with time)adapt its threshold based on recent samples

Third argumentassumption: make no assumption about disk access patternchoose thresholds randomly

Load-change StrategiesCommon strategy

changing the configuration or usage of the disk cacheincrease the cache sizechanging the dirty block timeout period (0 to 30s)add file names and attributes caching

Prefetchingdisk cache is filled with data that will likely be needed in the future before it is spun down

Reduce paging activityreduce working set sizesimprove memory access locality

Adaptation Strategies(1/4)Flash memory

advantage:• nonvolatile and low power consumption• reading as fast as DRAM• no seek latency

disadvantage:• poorer write performance (10-100 times slow

er than hard disk)• data cannot be overwritten without erasing t

he entire segment containing it

Adaptation Strategies(2/4)Flash memory as disk cache

effective as a second-level cache• most writes would be flushes from first-

level and asynchronous

problem: • cannot be overwritten without erasing the

segment

sol:• ensure there is always a segment with

free space for writing

Adaptation Strategies(3/4)Flash memory as disk

No seek latencydisk cache is no longer important

• except to be used as a write buffer (asynchronous write)

problem: • cannot be overwritten without erasing the segment

sol:Log-structured file system

• new data does not overwrite old data but appended to a log

Adaptation Strategies(4/4)Wireless network as disk

no power consumption of the storage deviceproblem:

• increased power consumption of the wireless communication

• higher latency for file system accesses (wireless bandwidth)

Several modelstransmit data access requests to, and receive data from, a serverstorage device as a large cache for the server file systemperform all processing on a server

The limiting factor in all of these cases is network bandwidth

IntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

Hardware FeaturesSlow down the clock

without changing the voltage is not usefulenergy consumption is invariant with clock speed

Selectively shut off functional unitssuch as the floating-point unit

Shut down processor operation altogetherreturn to full power when the next interrupt occurs

2

2 /1

CVE

PtEftfCVP

Transition StrategiesWhen the processor can turn off

from the current status of all processes• when all processes are blocked

When it can change the processor speedslowing the clock is accompanied by reducing the voltage

problem:• make an appropriate trade-off between energy

reduction and performance

Adjust the processor speed graduallyinterval-based approach attempting to complete all processor work by the end of each interval

Load-change StrategiesWhen the processor can turn off

reduce the time tasks take (efficient OS)use lower-power instructions (energy aware compilers)reduce the number of unnecessary tasks

• when an application is idle, it will “busy-wait” for an event instead of blocking (period satisfies conditions)

When functional units can turn offcompiler that clusters together several uses of a pipelined functional unitcompiler that generates instructions using functional units that do not get power-managed

IntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

Hardware FeaturesFive operating modes:

transmitreceiveidle • no tx/rx • transceiver is still powered and ready to rx or txsleep• similar to idle but transceiver powered downoff

Change transmission powerPower consumption depends on the distance to the receiver

Transition StrategiesWhen to enter sleep mode

strategies similar to those for hard disks• inactivity threshold methods

wireless device does not have the large mechanical componentswireless device should periodically exit sleep mode to send signal to the server (alive)

Effects by changing transmission powerincreased battery lifetimelower bit error rate for neighborshigher bit error rate for one’s own transmissions

When to change powerlocal strategyglobal strategy

Load-change StrategiesWhen sleep mode is used

increase the amount of time it can spend sleepingreduce network usage altogether

• Ex: compress TCP/IP headers; stop transmission when channel is bad

Use MAC protocolsleep when it is certain that no data will arrive for iteach mobile unit only be awake during the broadcast of the traffic schedule

When it can change transmission powermitigate the side effects of increased bit error rate

IntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

Hardware FeaturesEnergy-saving features

Backlight• brightness level• turn it off

Display• switch from color to monochrome• reduce update frequency• turn it off

Software StrategiesWhen to turn off

after a certain period of time has passed with no user inputmake it progressively dimmer

When to switch mode or reduce frequency

Current display mode are not visually important to the user

IntroductionGeneral IssuesStrategies for Components

Secondary storageProcessorWireless communication devicesDisplay and backlight

Conclusions

ConclusionsPower-saving three categories of software strategies

transitionload-changeAdaptation

trade-off between energy savings and performance

hardware features are rarely complete

not only reducing the power consumption, but also introducing lower-power, lower-functionality modes