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Abstract
This paper explores the complexity of implementing directory protocols by examining their mechanisms - primitive operations on directories, caches, and network interfaces. We compare the following protocols: Dir1 B, Dir4 B, Dir4 NB, Dirn NB, Dir1 SW and an improved version of Dir1 SW (Dir1 SW+). The comparison shows that the mechanisms and mechanism sequencing of Dir1 SW and Dir1 SW+ are simpler than those for other protocols. We also compare protocol performance by running eight benchmarks on 32 processor systems. Simulations show that Dir1 SW+'s performance is comparable to more complex directory protocols. The significant disparity in hardware complexity and the small difference in performance argue that Dir1 SW+ may be a more effective use of resources. The small performance difference is attributable to two factors: the low degree of sharing in the benchmarks and Check-In/Check-Out (CICO) directives.