An evaluation of directory schemes for cache coherence

The problem of cache coherence in shared-memory multiprocessors is addressed using two basic approaches: directory schemes and snoopy cache systems. Directory schemes for cache coherence are potentially attractive in large multiprocessor systems that are beyond the scaling limits of the snoopy cache schemes. Slight modifications to directory schemes can make them competitive in performance with snoopy cache schemes for small multiprocessors. Trace-driven simulation, using data collected from several real multiprocessor applications, is used to compare the performance of standard directory schemes, modifications to these schemes, and snoopy cache protocols. In addition, the simulations show that most blocks that are written into are present in only a small number of other caches, which makes broadcast invalidates inefficient. This result suggests that a directory structure that stores with each block only a small number of pointers to caches containing the block is sufficient.<>