Evaluating The Performance Of Four Snooping Cache Coherency Protocols

Write-invalidate and write-broadcast coherency protocols have been criticized for being unable to achieve good bus performance across all cache configurations. In particular, write-invalidate performance can suffer as block size increases; and large cache sizes will hurt write-broadcast. Read-broadcast and competitive snooping extensions to the protocols have been proposed to solve each problem. Our results indicate that the benefits of the extensions are limited. Read-broadcast reduces the number of invalidation misses, but at a high cost in processor lockout from the cache. The net effect can be an increase in total execution cycles. Competitive snooping benefits only those programs with high per-processor locality Of reference to shared data. For programs characterized by inter-processor contention for shared addresses, competi- tive snooping can degrade performance by causing a slight increase in bus utilization and total execution time.