1. Software Strategies for Portable Computer Energy Management IEEE Personal Communications 1998 Presented by Hsu Hao Chen

2. Outline Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

3. Introduction(1/2) Limiting the energy consumption of computers, especially portables, is becoming increasingly important. Goal reduce 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 functionality discuss software techniques for taking advantages of low-power hardware

4. Introduction(2/2) Power-saving three categories of software strategies transition issue load-change issue adaptation issue

5. Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

6. Strategy Types Transition strategies (prediction strategies) determining when to switch to low-power modes knowledge about its mode characteristics information about its future functionality requirements Load-change strategies how to modify the load on a component in order to increase its use of low-power modes Adaptation strategies how to create software that allows components to be used in power-saving ways

7. Levels of Energy Management Application Level User Level OS Level Component Level Abstraction Lack information about the overall workload Lack certain useful information about the state of the machine Application-aware adaptation

8. Strategy Evaluation and Power Budget Strategy Evaluation Power Budget mechanical motion and light generation seem inherently more power intensive than digital circuits.

9. Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

10. Hardware Features Most hard disks have five power modes: activeidlestandbysleepoff r/wcontrolspincacher/wcontrolspincache r/w controlspincache r/w controlspincache r/ w control spincache

11. Transition Strategies Disks can be of three kinds: deciding when to go to sleep mode deciding when to go to standby mode (no studies) deciding when to go to off mode Common strategy use inactivity thresholds as low as 1-10 seconds (Researchers discovered ) enter sleep mode after a fixed inactivity threshold (3-5min) Problems make the user wait more often for the disk to spin-up cause disk failure from frequent spin up/down cycles It is very difficult to predict accurate disk access pattern due to wide-varying of human activities

12. Changing Inactivity Threshold First argument assumption: disk access interarrival times are drawn from some unknown stationary distribution keep track of all interarrival times in order to deduce the best threshold Second argument assumption: disk access interarrival time distribution is nonstationary (i.e. changing with time) adapt its threshold based on recent samples Third argument assumption: make no assumption about disk access pattern choose thresholds randomly

13. Load-change Strategies Common strategy changing the configuration or usage of the disk cache increase the cache size changing the dirty block timeout period (0 to 30s) add file names and attributes caching Prefetching disk cache is filled with data that will likely be needed in the future before it is spun down Reduce paging activity reduce working set sizes improve memory access locality

14. 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 slower than hard disk) data cannot be overwritten without erasing the entire segment containing it

15. 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

16. Adaptation Strategies(3/4) Flash memory as disk No seek latency disk 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

17. Adaptation Strategies(4/4) Wireless network as disk no power consumption of the storage device problem: increased power consumption of the wireless communication higher latency for file system accesses (wireless bandwidth) Several models transmit data access requests to, and receive data from, a server storage device as a large cache for the server file system perform all processing on a server The limiting factor in all of these cases is network bandwidth

18. Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

19. Hardware Features Slow down the clock without changing the voltage is not useful energy consumption is invariant with clock speed Selectively shut off functional units such as the floating-point unit Shut down processor operation altogether return to full power when the next interrupt occurs

20. Transition Strategies When the processor can turn off from the current status of all processes when all processes are blocked When it can change the processor speed slowing the clock is accompanied by reducing the voltage problem: make an appropriate trade-off between energy reduction and performance Adjust the processor speed gradually interval-based approach attempting to complete all processor work by the end of each interval

21. Load-change Strategies When 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 off compiler that clusters together several uses of a pipelined functional unit compiler that generates instructions using functional units that do not get power-managed

22. Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

23. Hardware Features Five operating modes: transmit receive idle no tx/rx transceiver is still powered and ready to rx or tx sleep similar to idle but transceiver powered down off Change transmission power Power consumption depends on the distance to the receiver

24. Transition Strategies When to enter sleep mode strategies similar to those for hard disks inactivity threshold methods wireless device does not have the large mechanical components wireless device should periodically exit sleep mode to send signal to the server (alive) Effects by changing transmission power increased battery lifetime lower bit error rate for neighbors higher bit error rate for one ’ s own transmissions When to change power local strategy global strategy

25. Load-change Strategies When sleep mode is used increase the amount of time it can spend sleeping reduce network usage altogether Ex: compress TCP/IP headers; stop transmission when channel is bad Use MAC protocol sleep when it is certain that no data will arrive for it each mobile unit only be awake during the broadcast of the traffic schedule When it can change transmission power mitigate the side effects of increased bit error rate

26. Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

27. Hardware Features Energy-saving features Backlight brightness level turn it off Display switch from color to monochrome reduce update frequency turn it off

28. Software Strategies When to turn off after a certain period of time has passed with no user input make it progressively dimmer When to switch mode or reduce frequency Current display mode are not visually important to the user

29. Introduction General Issues Strategies for Components Secondary storage Processor Wireless communication devices Display and backlight Conclusions

30. Power-saving three categories of software strategies transition load-change Adaptation 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