Mechanisms For Cooperative Shared Memory

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: Dir₁B, Dir₄B, Dir₄NB, Dir_nNB[2], Dir₁SW[9] and an improved version of Dir₁SW (Dir₁SW⁺). The comparison shows that the mechanisms and mechanism sequencing of Dir₁SW and Dir₁SW⁺ are simpler than those for other protocols. We also compare protocol performance by running eight benchmarks on 32 processor systems. Simulations show that Dir₁SW⁺s performance is comparable to more complex directory protocols. The significant disparity in hardware complexity and the small difference in performance argue that Dir₁SW⁺ 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 [9].