Boosting the performance of hybrid snooping cache protocols

Previous studies of bus-based shared-memory multiprocessors have shown hybrid write-invalidate/write-update snooping protocols to be incapable of providing consistent performance improvements over write-invalidate protocols. We analyze the deficiencies of hybrid snooping protocols under release consistency, and show how these deficiencies can be dramatically reduced by using write caches and read snarfing. Our performance evaluation is based on program-driven simulation and a set of five scientific applications with different sharing behaviors including migratory sharing as well as producer-consumer sharing. We show that a hybrid protocol, extended with write caches as well as read snarfing, manages to reduce the number of coherence misses by between 83% and 95% as compared to a write-invalidate protocol for all five applications in this study. In addition the number of bus transactions is reduced by between 36% and 60% for four of the applications and by 9% for the fifth application. Because of the small implementation cost of the hybrid protocol and the two extensions, we believe that this combination is an effective approach to boost the performance of bus-based multiprocessors.