1. JouleTrack - A Web Based Tool for Software Energy Profiling Amit Sinha and Anantha Chandrakasan Massachusetts Institute of Technology June 19, 2001

2. Outline Motivation for fast software energy profiling Experimental setup – StrongARM and SH-4 Current profiling measurements and implications  Instruction level  Program level  Models Leakage energy estimation methodology  Processor leakage model  Leakage vs. switching JouleTrack design Conclusions

3. Motivation Proliferation of portable devices  50X increase in μP power vs. 4X in battery life  Battery technology lags behind Programmable solutions preferred  Fast software profiling techniques required

4. Scenarios Integrated Development Environments Energy aware software Portable devices downloading apps Predictive energy estimation Wireless sensor networks Lifetime estimation Energy-Quality tradeoffs

5. Software Energy Estimation [Tiwari96] proposed instruction level analysis Disadvantages  Tedious instruction level analysis  Large simulation time and storage requirement  Very fine grained Macro-estimation? Processor Memory AA Power Supply

6. StrongARM DVS Experimental Setup Intel StrongARM SA-1100 Brutus platform Hitachi SH-4 based SH7750 platform Freq59 – 206 MHz V DD 0.8 – 1.5 V SH7750

7. SA-1100 Instruction Current Little variation in instruction currents! 0 5 10 15 20 25 30 35 40 45 Power (%) CacheGCLKI/O,PLL [Montanaro, JSSC ‘96] ControlEBOX Supply V DD 1.5 V Frequency206 MHz Load/Store 0.196 A ALU0.178 A

8. SH-4 Instruction Currents Similar variation with MACs and Branches consuming more current

9. Program Current for SA-1100 Current almost independent of program! First order model, I = f(V DD,frequency)

10. Second Order Estimation Determine energy differentiated instruction classes and weights for those classes ClassWeight Instruction2.17 Sequential Memory access0.03 Non-sequential Memory access1.24 Internal0.84 Measure statistics for those classes at runtime Program current = Weighted average Error < 3% SA-1100 Example

11. Model Calibration Benchmark Programs Determine Weights Measure Statistics Execute Measure Current

12. First and Second Order Models Second order model reduces average error to 2.8% (based on 66 test programs)

13. Voltage Frequency Variation E switch = C tot V DD 2 E leak = V DD I 0 exp(V dd /nV T ) t Optimal

14. Leakage Model Leakage currents account for 10% of energy dissipation Leakage behavior is exponential with supply voltage I 0 = 1.2mA, n = 21.3 for the StrongARM FFT Execution TimeSupply Voltage (V) Total Charge flow Leakage Current (mA) I leak = I 0 exp(V dd /nV T )

15. Explanation of Exponential Model Processor leakage model can be explained from transistor subthreshold leakage model Model error < 6% Vdd (V) I leak (mA)Error (%) MeasuredModel 1.520.4120.101.5 1.416.3516.65-1.8 1.313.2613.80-4.0 1.212.0711.435.3 1.19.399.47-0.9 1.07.967.851.4 0.96.396.53-1.7  8.12 0 1 0 eV L W CA eAeI T eff ox V V nV VVVV T DS T SthGS                 V DD I1I1 I2I2 I3I3 [Gu96]

16. Leakage energy component can dominate switching energy for high supply voltage operation! Isolating Leakage Energy

17. Low Duty Cycle Effects Leakage occurs at all times! D = 100%  Leakage is 10% D = 10%  Leakage > 50% Just in time processing will reduce leakage energy Switching Energy Leakage Energy Time Energy TT1T1 Duty Cycle : D = T 1 /T Task completes Idle Mode

18. JouleTrack Design Web Server Common Gateway Interface www Simulator Compiler Linker Libraries Energy Estimation Average Currents, Weights Remote User source.c source.exe Cycle counts Statistics Errors Energy Profile Logs

19. Timing Estimation Engine Cycle Accurate Simulator Core Execution Statistics Memory Timing 00000000 8000 SRAM 4 rw 1/1 1/1 00008000 8000 ROM 2 r 100/100 100/100 00010000 8000 DRAM 2 rw 150/100 150/100 Tracer Co-processor Profiler Memory Models OS Models Memory Map Executable Image Clock Speed RDI

20. JouleTrack Snapshot http://dry-martini.mit.edu/JouleTrack 4000 hits since Sept-2000

21. Conclusions Fast macro-level program current estimation demonstrated  No need for elaborate trace profiling  Can be incorporated in an energy aware application development environment Very little instruction current variation because of common overheads Leakage currents becoming significant  10% at 100% duty cycle on StrongARM  Leakage measurement technique presented