A hierarchical directory scheme for large-scale cache-coherent multiprocessors

Cache coherence problem is a major design issue for shared-memory multiprocessors. As the system size scales, traditional bus-based snoopy cache coherence schemes are no longer adequate. Instead, the directory-based scheme is a promising approach to deal with the large-scale cache coherence problem. However, the storage overhead of directory schemes often becomes too prohibitive as the system size increases. The paper proposes the hierarchical full-map directory to reduce the storage requirement while still achieving satisfactory performance. The key point is to exploit the inherent geographical interprocessor locality among shared data in the parallel programs. Trace-driven evaluations show that the performance of the proposed scheme compares competitively to the full-map directory scheme, while reducing the storage overhead by over 90%. The proposed hierarchical full-map directory scheme seems to be a promising hardware approach for handling cache coherence in the design of future large-scale multiprocessor memory systems