Using prediction to accelerate coherence protocols

Abstract

Most large shared-memory multiprocessors use directory protocols to keep per-processor caches coherent. Some memory references in such systems, however suffer long latencies for misses to remotely-cached blocks. To ameliorate this latency, researchers have augmented standard coherence protocols with optimizations for specific sharing patterns, such as read-modify-write, producer-consumer and migratory sharing. This paper seeks to replace these directed solutions with general prediction logic that monitors coherence activity and triggers appropriate coherence actions. This paper takes the first step toward using general prediction to accelerate coherence protocols by developing and evaluating the Cosmos coherence message predictor. Cosmos predicts the source and type of the next coherence message for a cache block using logic that is an extension of Yeh and Patt's two-level PAp branch predictor. For five scientific applications running on 16 processors, Cosmos has prediction accuracies of 62% to 93%. Cosmos' high prediction accuracy is a result of predictable coherence message signatures that arise from stable sharing patterns of cache blocks.