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 snoop-ing extensions to the protocols have been proposed to solve each problem. Our results indicate that the benefits of the exten-sions am 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. 1