A mechanism for speculative memory accesses following synchronizing operations

In order to reduce the overhead of synchronizing operations of shared memory multiprocessors, this paper proposes a mechanism, named specMEM, to execute memory accesses following a synchronizing operation speculatively before the completion of the synchronization is confirmed. A unique feature of our mechanism is that the detection of speculation failure and the restoration of computational state on the failure are implemented by a small extension of coherent cache. It is also remarkable that operations for speculation on its success and failure are performed in a constant time for each independent of the number of speculative accesses. This is realized by implementing a part of cache tag for cache line state with a simple functional memory. This paper also describes an evaluation result of specMEM applied to barrier synchronization. Performance data was obtained by simulation running benchmark programs in SPLASH-2. We found that the execution time of LU decomposition, in which the length of period between a pair of barriers significantly varies because of the fluctuation of computational load, is improved by 13%