An Evaluation of Stratified Sampling of Microarchitecture Simulations

Recent research advocates applying sampling to accelerate microarchitecture simulation. Simple random sampling offers accurate performance estimates (with a high quantifiable confidence) by taking a large number (e.g., 10,000) of short performance measurements over the full length of a benchmark. Simple random sampling does not exploit the often repetitive behaviors of benchmarks, collecting many redundant measurements. By identifying repetitive behaviors, we can apply stratified random sampling to achieve the same confidence as simple random sampling with far fewer measurements. Our oracle limit study of optimal stratified sampling of SPEC2K benchmarks demonstrates an opportunity to reduce required measurement by 43x over simple random sampling. Using our oracle results as a basis for comparison, we evaluate two practical approaches for selecting strata, program phase detection and IPC profiling. Program phase detection is attractive because it is microarchitec- ture independent, while IPC profiling directly minimizes stratum variance, therefore minimizing sample size. Unfortunately, our results indicate that: (1) program phase stratification falls far short of optimal opportunity, (2) IPC profiling requires expensive microarchitecture- specific analysis, and (3) both methods require large sampling unit sizes to make strata selection feasible, offsetting reductions in sample size. We conclude that, without better stratification approaches, stratified sampling does not provide a clear advantage over simple random sampling.