Dynamic self-invalidation: reducing coherence overhead in shared-memory multiprocessors

The paper introduces dynamic self-invalidation (DSI), a new technique for reducing cache coherence overhead in shared-memory multiprocessors. DSI eliminates invalidation messages by having a processor automatically invalidate its local copy of a cache block before a conflicting access by another processor. Eliminating invalidation overhead is particularly important under sequential consistency: where the latency of invalidating outstanding copies can increase a program´s critical path. DSI is applicable to software, hardware, and hybrid coherence schemes. We evaluate DSI in the context of hardware directory-based write-invalidate coherence protocols. Our results show that DSI reduces execution time of a sequentially consistent full-map coherence protocol by as much as 41%. This is comparable to an implementation of weak consistency that uses a coalescing write-buffer to allow up to 16 outstanding requests for exclusive blocks. When used in conjunction with weak consistency DSI can exploit tear-off blocks-which eliminate both invalidation and acknowledgment messages-for a total reduction in messages of up to 26%.