The performance of three directory-based cache- coherence protocols; strict request-response, intervention forwarding and reply forwarding are evaluated via simulation on the SOME-Bus, which is a fiber-optic interconnection network supporting DSM. The simulated system contains 64 nodes, each of which has a processor, cache controller, directory controller and output channel. Simulations have been conducted for each protocol to measure average processor utilization and average network latency for varying values of DSM parameters such as the ratio of the mean channel service time to mean thread run time (T/R), probability of a cache block being in modified state {P(M)}, the fraction of write misses {P(W)} and under different traffic patterns. The results reveal that the performance of all protocols decreases under all traffic patterns as P(W), P(M) or T/R increases. The effect of P(W) on the performance of the protocols reduces as P(M) increases. Reply forwarding performs the best for high P(M) values, intervention forwarding yields the best performance for low P(M) and high P(W) values and strict request-response is the best protocol under hot-region (HR) traffic.

Abstract: "Directory schemes have been proposed to solve the cache coherence problem for large-scale multiprocessor systems. Most of previous studies concentrated on cost reduction for the design of directory schemes. With scalable directory design, there are various design parameters that affect its performance. Their impact is impossible to predict. In this paper, we evaluate the effect of these parameters on the performance of directory schemes concentrating on shared data, including cache organization, directory protocols, scalability and memory latency. We also analyze the resource contention and coherence delays of directory schemes and discuss possible improvements."