Two economical directory schemes for large-scale cache coherent multiprocessors

Cache coherence problem is a major issue in the design of shared-memory multiprocessors. As the number of processors grows, traditional bus-based snoopy schemes for cache coherence are no longer adequate. Instead, the directory-based scheme is a promising alternative for the large-scale cache coherence problem. However, the storage overhead of (full-map) directory scheme may become too prohibitive as the system size goes up. This paper presents two distributed directory schemes, the tree directory and the hierarchical full-map directory , to deal with the storage overhead problem. Preliminary trace-driven evaluations show that the performance of our schemes compares favorably to the full-map directory scheme, while reducing the storage overhead by over 90%. These two schemes should lend themselves to the design and implementation of large-scale cache coherent multiprocessors.