Analysis of Sharing Overhead in Shared Memory Multiprocessors

A cache memory contributes in both hiding memory latency and reducing the traffic on the processor interconnection network of shared memory multiprocessors but it causes the coherence problem. A coherence protocol [Tomasevic93] is required in order to guarantee the coherence of the cached copies. An adequate choice of the coherence protocol is critical for performance. In fact, when the number of nodes exceeds a critical value, the processor interconnection network reaches a saturation condition, due to both cache misses and coherence operations. Three main classes of coherence protocols are write-update (WU), write-invalidate (WI), and hybrid. A WU protocol updates the remote copies on each write involving on a shared copy. Whereas, a WI protocol invalidates previously remote copies in order to avoid updating them. A hybrid protocol uses both WU and WI strategies to combine the best aspect of each one. The frequency and the pattern of accesses to shared copies influence coherence overhead. Since access pattern to shared data varies from application to application; neither WI nor WU is the better strategy for maintaining cache coherence in all cases. Results [Veenstra94] show that, although the hybrid protocol [Cox93, Prete90, Prete95b, Stenstrom93] does not offer any significant advantage over the best choice of pure protocols for a particular application, it may offer optimal performance over a wider range of applications than any single pure protocol. An optimal selection for the coherence protocol can be made by considering the traffic induced by the two approaches in case of different sharing. The coherence overhead induced by a WU protocol is due to all the operations needed to update the remote copies. Whereas, a WI protocol invalidates remote copies and processors generate a miss on the access to the invalidated copy. Invalidation and block fetching (due to invalidation misses) contribute to the coherence overhead of WI protocols. By considering the cost for