1. Statistical Simulation of Superscalar Architectures using Commercial Workloads Lieven Eeckhout and Koen De Bosschere Dept. of Electronics and Information Systems (ELIS) Ghent University, Belgium CAECW’01, January 21, 2001

2. 2 Outline Introduction Statistical Simulation –Statistical profiling –Synthetic trace generation Methodology Evaluation Conclusion

3. 3 Introduction Architectural simulation –trace-driven or execution-driven –accurate –long simulation times –long traces to be stored Need for fast simulation techniques –take part of a full trace –analytical modeling –trace sampling –statistical simulation

4. 4 Goal Previous work used SPEC benchmarks to evaluate statistical simulation In this talk we use both commercial and scientific workloads –SPECint, SPECfp, system traces, multimedia, X graphics, database

5. 5 Statistical Simulation Three steps: –extract statistical profile from a program execution –generate synthetic trace from it –simulate on a trace-driven simulator Two major advantages: –statistical profile is more compact than full trace –fast simulation due to statistical nature  design space exploration in limited time

6. 6 statistical profile Statistical Simulation real trace (e.g. SPEC benchmark) branch profiling cache profiling instruction profiling branch statistics cache statistics instruction statistics synthetic trace generator synthetic trace trace-driven simulator

7. 7 Statistical Profiling Microarchitecture-independent statistics –instruction statistics Microarchitecture-dependent statistics –branch statistics –cache statistics Result: statistical simulation only to explore design options of processor core (cache and branch predictor are fixed)

8. 8 Statistical Profiling Instruction Statistics Instruction mix (13 classes) Number of register operands Age of register operands –probability that register operand was produced  instructions before it in the trace (only RAW) Memory dependencies –probability that load is memory-dependent on the  -th store before it in the trace (only RAW)

9. 9 Statistical Profiling Branch Statistics Six branch types –conditional branch, unconditional branch, call with offset, indirect jump, indirect call, return Distinction –branch prediction accuracy: refill pipeline on branch misprediction –branch target prediction accuracy: single- cycle bubble in pipeline on correct branch prediction but target misprediction

10. 10 Statistical Profiling Cache Statistics D-cache statistics –L1 D-cache miss rate –L2 D-cache miss rate I-cache statistics –L1 I-cache miss rate –L2 I-cache miss rate

11. 11 Synthetic Trace Generation Instruction-by-instruction through random number generation Determine instruction type number of operands age of register operands memory dependency branch behavior D-cache behavior I-cache behavior st add ld br mispredicted D-cache miss I-cache miss

12. 12 Methodology: microarchitecture Out-of-order processor –8 and 16 issue –windows of 64 and 128 instructions McFarling branch predictor ‘small’ cache configuration –8KB DM L1 I-cache, 8KB DM L1 D-cache, 64KB 2WSA unified L2 cache ‘large’ cache configuration –32KB DM L1 I-cache, 64KB 2WSA L1 D-cache, 512KB 4WSA unified L2 cache Access time –L1 I-cache (1 cycle), L1 D-cache (2 cycles), L2 cache (10 cycles), main memory (80 cycles)

13. 13 Methodology: benchmarks 8 SPECint95 benchmarks 5 SPECfp95 benchmarks (hydro2d, su2cor, swim, tomcatv, wave5) 8 IBS system traces (mpeg, jpeg, gs, verilog, gcc, sdet, nroff, groff) 4 MediaBench applications (g721, gs, gsm, mpeg2) 4 X graphics benchmarks (DooM, POVRay, Xanim, Quake) 2 TPC-D queries running on Postgres 6.3  ~ 200 million instructions / trace

14. 14 Evaluation IPC prediction error = IPC real trace - IPC synthetic trace IPC real trace IPC real trace = IPC when running real trace on trace-driven simulator IPC synthetic trace = IPC when running synthetic trace generated from the statistical profile of the real trace Simulation speed: s IPC /x IPC less than 1% after simulating 1 million instructions

15. 15 IPC prediction error (1) 157%135% -30% -20% -10% 0% 10% 20% 30% 40% hydro2d su2cor swim tomcatv wave5 mpeg jpeg gs verilog real_gcc sdet nroff groff g721_e gs gsm_e mpeg2 xanim xdoom xpovray xquake tpc-d.17 tpc-d.2 IPC prediction error SPECint95SPECfp95IBSMediaBenchX graphicsTPC-D li gcc compress go ijpeg vortex m88ksim perl 16-issue, 128-entry window, ‘small’ cache configuration high D-cache miss rate high D-cache miss rate

16. 16 IPC prediction error (2) -30% -20% -10% 0% 10% 20% 30% li gcc compress go ijpeg vortex m88ksim perl hydro2d su2cor swim tomcatv wave5 mpeg jpeg gs verilog real_gcc sdet nroffgroff g721_e gs gsm_e mpeg2 xanim xdoom xpovray xquake tpc-d.17 tpc-d.2 IPC prediction error SPECint95SPECfp95IBSMediaBenchX graphicsTPC-D 16-issue, 128-entry window, ‘large’ cache configuration

17. 17 IPC prediction error vs. static instruction count -40% -20% 0% 20% 40% 60% 80% 100% 120% 140% 160% 020000400006000080000100000120000140000160000 static instruction count (number of instructions executed at least once) IPC prediction error w = 64; i = 8; 'small' cache w = 128; i = 16; 'small' cache w = 64; i = 8; 'large' cache w = 128; i = 16; 'large' cache DooM Quake DooM Quake gs (IBS) gcc gcc (IBS) mpeg (IBS) groff mpeg (IBS) groff nroff jpeg (IBS) verilog sdet nroff jpeg (IBS) verilog sdet TPC-D vortex go vortex go

18. 18 Conclusion (1) Higher IPC prediction errors for applications with smaller static instruction count: –MediaBench applications –SPECfp95 benchmarks –2 X graphics benchmarks (POVRay and Xanim) –5 SPECint95 benchmarks

19. 19 Conclusion (2) Smaller IPC prediction errors for applications with larger instruction footprint: –IBS system traces –TPC-D traces –2 X graphics benchmarks (DooM and Quake) –3 SPECint95 benchmarks (go, gcc, vortex)  IPC prediction error between -1% and 25%

20. 20 Conclusion (3) Statistical simulation is a useful fast simulation technique for commercial workloads –due to higher variability in instructions –since commercial workloads have larger instruction footprint –which makes a statistical technique more powerful