Using destination-set prediction to improve the latency/bandwidth tradeoff in shared-memory multiprocessors

Destination set prediction can improve the latency/bandwidth tradeoff in shared memory multiprocessors. The destination set is the collection of processors that receive a particular coherence request. Snooping protocols send requests to the maximal destination set (i.e., all processors), reducing latency for cache to cache misses at the expense of increased traffic. Directory protocols send requests to the minimal destination set, reducing bandwidth at the expense of an indirection through the directory for cache to cache misses. Recently proposed hybrid protocols tradeoff latency and bandwidth by directly sending requests to a predicted destination set. We explore the destination set predictor design space, focusing on a collection of important commercial workloads. First, we analyze the sharing behavior of these workloads. Second, we propose predictors that exploit the observed sharing behavior to target different points in the latency/bandwidth tradeoff. Third, we illustrate the effectiveness of destination set predictors in the context of a multicast snooping protocol. For example, one of our predictors obtains almost 90% of the performance of snooping while using only 15% more bandwidth than a directory protocol (and less than half the bandwidth of snooping).