Invalidation bus optimization for multiprocessors using directory-based

Abstract

An optimization scheme for a directory-based cache coherence protocol for multistage interconnection network-based multiprocessors improves system performance by reducing network latency. The optimization scheme is scalable, targeting multiprocessor systems having a moderate number of processors. The modification of shared data is the dominant contributor to performance degradation in these systems. The directory-based cache coherence scheme uses an invalidation bus on the processor side of the network. The invalidation bus connects all the private caches in the system and processes the invalidation requests thereby eliminating the need to send invalidations across the network. In operation, a processor which attempts to modify data places an address of the data to be modified on the invalidation bus simultaneously with sending a store request for the data modification to the global directory and the global directory sends to the processor attempting to modify the data, in addition to the permission signal, a count of the number of invalidation acknowledgments the processor should receive.