Why invalidate protocol is used to maintain coherence?
Answers
Snoopy protocols are widely used for preserving of cache coherence in shared bus, shared memory multiprocessors. In this work, an attempt was made to improve their performance for parallel applications where sequential pattern of sharing prevails. The scheme introduced here, the WIP protocol, tries to achieve better data utilization, compared to the existing write-invalidate protocols by applying the principle of partial, word-based invalidation. The complete coherence mechanism is described in this paper. Both the analytical and the simulation methodology were used to evaluate the features of the proposed solution, and to compare it with the Berkeley and the Dragon protocols. Comparative evaluation is performed for a large variety of application and system oriented parameters. The results are also presented and discussed here. Implementation of the WIP in a cache memory unit is compared with the hardware complexity of the two considered protocols.