Exploring the Architecture of a Stream Register-Based Snoop Filter

Abstract

Multi-core processors have become mainstream; they provide parallelism with relatively low complexity. As true on-chip symmetric multiprocessors evolve, coherence traffic between cores is becoming problematic, both in terms of performance and power. The negative effects of coherence (snoop) traffic can be significantly mitigated through the use of snoop filtering. The idea is to shield each cache with a device that can eliminate snoop requests for addresses that are known not to be in the cache. This improves performance significantly for caches that cannot perform normal load and snoop lookups simultaneously. In addition, the reduction of snoop lookups yields power savings. This paper describes Stream Register snoop filtering, which captures the spatial locality of multiple memory reference streams in a small number of registers. We propose a snoop filter that combines Stream Registers with ”snoop caching”, a mechanism that captures the temporal locality of frequently-accessed addresses. Simulations of SPLASH-2 benchmarks on a 4-core multiprocessor illustrate tradeoffs and strengths of these two techniques. We show that their combination is most effective, eliminating 94% - 99% of all snoop requests using only a small number of stream registers and snoop cache lines.