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 LabDept. of Electrical and Computer Engineering

Carnegie Mellon University

‡Austin Research LaboratoryIBM

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

Energy Efficiency

...

power

time

activeE

t

Watts

activeE

idleE

t

Watts

Low frequency High frequency

• Majority of OS policies assume

• Not always the case!– When it is not the case?

– How do we determine this?

Assumption

<activeE

t

Watts

activeE

idleE

t

Watts

• 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

Outline

lowfE highfE

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.

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

m: slope

CPU active

CPU idle

Power

Frequency

Energy Consumption

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

activeE

idleE

tpower

time

0

500

1000

1500

2000

2500

3000

12 25 37 50 62 75 87 100

CPU performance (%)

J oules

Extra IdleSystem Active

Energy consumption (Pentium L1 cache read hit)

2490J

2591J174.3sec

Energy consumption (PPC L1 cache read hit)

0

50

100

150

200

66 133 200 266MHz

J oules

Extra IdleOthersSDRAMCPU

136J66.4sec

162J

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?

• 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

Outline

• 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:

Characterization

minf

minfT

minminmin fff PTE )( minff

ff

fT minmin

)min

1min( ff

fT

idleff

ffff

ff PTPTE )1()( minmin

minmin

– Power increases linearly with frequency– m: slope

• Is energy efficient??– True if – Depends on m

Critical Power Slope

)( minmin ffmPP ff

idleff

ffff

ff PTffmPTE )1()]()[( minminminmin

minmin

minfminff EE

• Use slope m to characterize system– Find hypothetical m for and call it

Critical Power Slope (CPS)

Critical Power Slope cont’d

minff EE

min

min

fPP

criticalidlefm

What does it mean?

Freq

Power

criticalm

minf

idleP

min

min

fPP

criticalidlefm

minfPidleP

criticalmm

criticalmm

• If– Energy efficient to run at higher freq.– Pentium

• If– Energy efficient to run at lower freq.– PowerPC

Implications of CPScriticalmm

criticalmm

028.%5.128481215

MHzWWcriticalm020.%5.12848

1530 MHzWWm

0038.6602.227.2 MHzWWcriticalm0043.66266

27.213.3 MHzMHz

WWm

minff EE

minff EE <

J.Pouwelse, K.Langendoen, and H. Sips, “Dynamic Voltage Scaling on a Low-Power Microprocessor”, MOBICOM2001

Voltage Scaling Processors (Strong Arm SA-1100)

• Look at every operating point at frequency

• If– Energy efficient at higher frequency than

• If– Energy efficient at lower frequency than

CPS for voltage scaling system

xf

fxPP fx

idlefxfx

criticalm

fx

critical

fx mm

fx

critical

fx mm xf

xf

Analysis on SA-1100

• Above 74MHz

• At 74MHz

• Below 74MHz

• Energy Inefficient below 74MHz!

001.0744612174

MHzmWmWMHz

criticalm

001.0597410612174

MHzMHz

mWmWMHzm

fx

critical

fx mm

fx

critical

fx mm

SummaryPower

Frequency

Power

Frequency

Power

Frequency

Pentium PowerPC

SA-1100 CPS: Characterizes the runtime trade-off of power management techniques

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