of the 8 th ACM International Conference on Supercomputing , July 1994 . An Evaluation of Directory Protocols for Medium - ScaleShared - Memory

This paper considers alternative directory protocols for providing cache coherence in shared-memory mul-tiprocessors with 32 to 128 processors, where the state requirements of Dir N may be considered tootate (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 rst to compare all these protocols with complete applications (and the rst evaluation of Tristate with a non-synthetic work-load). Results for three applications|ocean (one-dimensional 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 invali-dation messages as Dir N , (b) Gray-software sends 1.0 to 4.7 times as many messages as Dir N , making it better than Tristate, Gray-hardware, and Home, and (c) the choice between Dir i B, Coarse Vector, and Gray-software depends on whether one wants to optimize for few sharers (Dir i B), many sharers (Coarse Vector), or hedge one's bets between both alternatives (Gray-software).