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Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling

Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling. Akihiko Miyoshi † ,Charles Lefurgy ‡ , Eric Van Hensbergen ‡ , Ram Rajamony ‡ , Raj Rajkumar †. † Real-Time and Multimedia Systems Lab Dept. of Electrical and Computer Engineering Carnegie Mellon University.

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Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling

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  1. Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling Akihiko Miyoshi†,Charles Lefurgy‡, Eric Van Hensbergen ‡, Ram Rajamony ‡, Raj Rajkumar † † Real-Time and Multimedia Systems Lab Dept. of Electrical and Computer Engineering Carnegie Mellon University ‡Austin Research Laboratory IBM

  2. The Question • Operating Points • [600MHz,6V], [525MHz,4.2V],[450MHz,2.8V],[375MHz,2V],[300MHz, 1.7V], [225MHz,1.5V],[150MHz,1.45V] • Where should I operate(for energy efficiency)? • Dynamic Voltage Scaling (DVS) algorithms • Lowest performance without sacrificing user/application requirement • Why lowest performance is not always the best • Even for voltage scaling systems

  3. ... Watts Watts Energy Efficiency power Low frequency High frequency time

  4. Watts Watts Assumption • Majority of OS policies assume • Not always the case! • When it is not the case? • How do we determine this? <

  5. Outline • Motivation • <: not always true • How do we choose which operating points to use? • Measurement results • Analytical model: Critical Power Slope • Analysis on voltage scaling systems • Conclusion

  6. Power Management Techniques • Provides multiple operating points • [600MHz,6V],[450MHz,2.8V],[300MHz, 1.7V]…etc • Three empirical data points • Frequency Scaling • PowerPC 405GP • Clock Throttling • Pentium with ACPI • Voltage Scaling • Strong ARM SA-1100 • Note: We are not making any statement on the benefits of these techniques! • These are merely samples which real systems use to manage power.

  7. Basic Results • Runtime and frequency • CPU intensive workload: inverse relationship • Power and frequency • Frequency scaling, clock throttling processors • CPU active: linear relationship • CPU idle: constant CPU active Power m: slope CPU idle Frequency

  8. power time Energy Consumption • Compare energy consumption at different operating points • Same workload W • Same amount of time t

  9. Energy consumption (Pentium L1 cache read hit) 2591J 174.3sec 2490J

  10. Energy consumption (PPC L1 cache read hit) 162J 136J 66.4sec

  11. Measurement Results • Results consistent with different workloads • Register, L1 cache, memory, disk accesses • Web server (Pentium) • Pentium • Highest frequency always energy efficient • PowerPC • Lowest frequency always energy efficient • Why? • What happens on voltage scaling systems?

  12. Outline • Motivation • Which operating points should we consider? • Measurement results • Pentium: highest performance better • PowerPC: lowest performance better • Analytical model: Critical Power Slope • Analysis on voltage scaling systems • Conclusion

  13. Characterization • CPU intensive workload W • Frequency • Assume utilization of system = 1 • units of time to complete W • Energy consumed • At frequency • Time to compute W: • Remaining extra idle time:

  14. Critical Power Slope • Power increases linearly with frequency • m: slope • Is energy efficient?? • True if • Depends on m

  15. Critical Power Slope cont’d • Use slope m to characterize system • Find hypothetical m for and call it Critical Power Slope (CPS)

  16. What does it mean? Power Freq

  17. Implications of CPS < • If • Energy efficient to run at higher freq. • Pentium • If • Energy efficient to run at lower freq. • PowerPC

  18. Voltage Scaling Processors (Strong Arm SA-1100) J.Pouwelse, K.Langendoen, and H. Sips, “Dynamic Voltage Scaling on a Low-Power Microprocessor”, MOBICOM2001

  19. CPS for voltage scaling system • Look at every operating point at frequency • If • Energy efficient at higher frequency than • If • Energy efficient at lower frequency than

  20. Analysis on SA-1100 • Above 74MHz • At 74MHz • Below 74MHz • Energy Inefficient below 74MHz!

  21. Summary Power Pentium Power PowerPC Frequency Frequency Power SA-1100 CPS: Characterizes the runtime trade-off of power management techniques Frequency

  22. Conclusion • Which operating points should we consider? • Traditional DVS algorithms attempt to go to lowest frequency • Not always the best choice • Critical Power Slope • Identifies energy inefficient operating points • Can be used to inform OS (DVS algorithms) of operating points it should not consider

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