Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling

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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
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The Question

• Operating Points
• 600MHz,6V, 525MHz,4.2V,450MHz,2.8V,375MHz,2
  V,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

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Energy Efficiency
power
Low frequency
High frequency
time
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Assumption

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

lt
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Outline

• Motivation
• lt 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

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Power Management Techniques

• Provides multiple operating points
• 600MHz,6V,450MHz,2.8V,300MHz, 1.7Vetc
• 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.

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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
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Energy Consumption

• Compare energy consumption at different operating
  points
• Same workload W
• Same amount of time t

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Energy consumption (Pentium L1 cache read hit)
2591J 174.3sec
2490J
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Energy consumption (PPC L1 cache read hit)
162J
136J 66.4sec
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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?

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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

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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

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Critical Power Slope

• Power increases linearly with frequency
• m slope
• Is energy efficient??
• True if
• Depends on m

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Critical Power Slope contd

• Use slope m to characterize system
• Find hypothetical m for
  and call it Critical Power Slope (CPS)

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What does it mean?
Power
Freq
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Implications of CPS

• If
• Energy efficient to run at higher freq.
• Pentium
• If
• Energy efficient to run at lower freq.
• PowerPC

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

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Analysis on SA-1100

• Above 74MHz
• At 74MHz
• Below 74MHz
• Energy Inefficient below 74MHz!

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