Using Partial Tag Comparison in Low-Power Snoop-Based Chip Multiprocessors

In this work we introduce power optimizations relying on partial tag comparison (PTC) in snoop-based chip multiprocessors. Our optimizations rely on the observation that detecting tag mismatches in a snoop-based chip multiprocessor does not require aggressively processing the entire tag. In fact, a high percentage of cache mismatches could be detected by utilizing a small subset but highly informative portion of the tag bits.Based on this, we introduce a source-based snoop filtering mechanism referred to as S-PTC. In S-PTC possible remote tag mismatches are detected prior to sending the request. We reduce power as S-PTC prevents sending unnecessary snoops and avoids unessential tag lookups at the end-points. Furthermore, S-PTC improves performance as a result of early cache miss detection.S-PTC improves average performance from 2.9% to 3.5% for different configurations and for the SPLASH-2 benchmarks used in this study. Our solutions reduce snoop request bandwidth from 78.5% to 81.9% and average tag array dynamic power by about 52%.