An Evaluation of Directory Protocols for Medium-Scale Shared-Memory Multiprocessors *

This paper considers alternative directory protocols for providing cache coherence in shared-memory multiprocessors with 32 to 128 processors, where the state requirements of DirN may be considered too large. We consider Dir i B, i = 1; 2; 4, DirN , Tristate (also called superset), Coarse Vector, and three new protocols. The new protocols---Gray-hardware, Gray-software, Home---are optimizations of Tristate that use gray coding to favor near-neighbor sharing. Our results are the first to compare all these protocols with complete applications (and the first evaluation of Tristate with a non-synthetic workload) . Results for three applications---ocean (onedimensional sharing), appbt (three-dimensional sharing) , and barnes (dynamic sharing)---for 128 processors on the Wisconsin Wind Tunnel show that (a) Dir 1 B sends 15 to 43 times as many invalidation messages as DirN , (b) Gray-software sends 1.0 to 4.7 times as many messages as DirN , making it better than Tristate, Gray-hardware, a..