Power Management for Mobile Operating System

8/2/2019 Power Management for Mobile Operating System

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Hans Joshep V. OBrien

Lanie M. Arco

Power Management for MobileOperating System


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Introduction Batteries

Reducing physical dimension

If developers successfully reduce the basic

consumption of mobile computing devices, usersmay have the bonus of either retaining the chargecharacteristics of the battery even the batterydimension is reduced and also increase thecharge time of the battery.


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Introduction There are components in mobile computing

devices that are responsible for powerconsumption, these includes the physicalcomponents.


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Power Consumption of CPU CPUs clock rate

is related to the power consumed by the CPU, thesupply voltage, and the capacitance of the devicesbeing switched (e.g. the transistors).

- when the CPUs clock rate decreases the powerconsumption also deceases, clock rate decreasesbecause of the switching characteristics of the logicgates (Gregory F. Welch, 1994).


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Scheduling for Reduced CPU Energy

CPUs power consumption is significant in mobilecomputing device.

If the CPU clock frequency and supply voltagecan be controlled, linear and quadratic savings inpower can be realized.

Because lowering the supply voltage results in aquadratic power savings, this is the authors

preferred method for power reduction.


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Scheduling for Reduced CPU Energy

However, the supply voltage cannot be lowered(dynamically) without also reducing the clockspeed.

The authors consider the CPU clock rate to belinearly adjusted with the supply voltage.


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Method Three algorithms for adjusting the CPU clock

speed under the control of the operating system.

The evaluation of the algorithms wasaccomplished through trace-driven simulations.


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Results The three algorithms basic approach is to

balance the usage between the periodic bursts ofhigh CPU utilization and the remaining idle time ofthe CPU.

For instance, lets assume that we have a job that

has a CPU burst time of 10 milliseconds usage atthe full CPU clock speed and followed by another10 milliseconds idle time.


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Results If it is possible that that job can be replaced by a

20 milliseconds period of CPU usage at half thenormal clock speed, without affecting the useradversely, then such a scheme can be used to

conserve power (Welch, G. 1994).


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Results All three algorithms rely on the assumption that

the operating system sleepevents that normallyresult in CPU idle time and can be classified intotwo categories:

- hard and soft sleep events.


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Results Hard sleep events

events result in idle time during which the CPUspeed cannot be reduced.

Soft sleep events

- result in idle time during which the CPUspeed can be reduced


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Results The authors classify of CPU idle time because

they want to be fair about assessments, the

authors want to know which idle times weresubject for use in balancing periods of high and

low activity.


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Discussions Three algorithms by Weiser et al. 1994

OPT

FUTURE

PAST


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Discussions

OPT Algorithm is an impractical and undesirable algorithm. It is

completely optimistic (and impractical) in the sensethat it assumes complete knowledge of the future

work to be done in an interval (Welch, G. 1994).The said algorithm is undesirable because itrigorously affect the run times of user tasks anddisregarding importance of events such as

keystroke response or network communications.


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Discussions

FUTURE Algorithm- is similar to OPT because it also assumes future

task to be done but it does so for only shortwindows of time, and optimizes over only those

windows.


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Discussions FUTURE is unrealistic because it peers into the

future.

Unlike OPT, it is practical because it onlyoptimizes over short windows, therefore the effect

and impact on time-critical work is minimized.


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Discussions PAST

Unlike OPT and FUTURE algorithms PASTalgorithm is a realistic (practical) version of thesealgorithms.

PAST algorithm is realistic because instead ofpeering into the future, it looks to a short window inthe past for information about CPU usage.


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Discussions

- It is desirable because like FUTURE it onlymonitors activity over a short window of the trace.But of the disadvantage of this algorithm is that itassumes the activity in the next (future) window

will be like that in the current (past) window.


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Discussions

- it is indeed more profitable (in terms of powersavings) to spread out work when possible overperiods of slower CPU clock rates, rather thancontending with bursts of high-speed activity

followed by wasted idle time.


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Discussions

- The authors also concluded that in theirsimulations a PAST implementation with a 50 mswindow would have saved up to 50% of thepower in a 3.3 volt CPU


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Conclusion The development and rise of mobile computing

has introduced new challenges and issues for thedesigners of computers and computer operatingsystems.

These techniques together focus on what aretypically the main causes in terms of powerconsumption in mobile computing: CPU/memorydevices.


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Conclusion While hardware advances have and will likely

continues to reduce the power consumption ofthese devices, efficient operating systemtechniques significantly may reduce power

consumption without considerably affecting theperceived performance.

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Power Management for Mobile Operating System