Adaptive Cache Coherency For Detecting Migratory Shared Data

Parallel programs exhibit a small number of distinct data-sharing patterns. A common data-sharing pattern, migratory access, is characterized by exclusive read and write access by one processor at a time to a shared datum. We describe a family of adaptive cache coherency protocols that dynamically identify migratory shared data in order to reduce the cost of moving them. The protocols use a standard memory model and processor-cache interface. They do not require any compile-time or run-time software support. We describe implementations for bus-based multiprocessors and for shared-memory multiprocessors that use directory-based caches. These implementations are simple and would not significantly increase hardware cost. We use trace- and execution-driven simulation to compare the performance of the adaptive protocols to standard write-invalidate protocols. These simulations indicate that, compared to conventional protocols, the use of the adaptive protocol can almost halve the number of inter-node messages on some applications. Since cache coherency traffic represents a larger part of the total communication as cache size increases, the relative benefit of using the adaptive protocol also increases.