In this paper we introduce a classification of misses in shared-memory multipro- cessors based on inter processor communication. We identify the set of essential misses, i.e., the smallest set of misses necessary for correct execution. Essential misses include cold misses and true sharing misses. All other misses are useless misses and can be ignored without affecting program execution. Based on the new classification we evaluate miss reduction techniques in hard- ware, based ondelayingand combining invalidations. We compare the effectiveness of five different protocols for combining invalidations leading to useless misses for cache- based multiprocessors and for multiprocessors with virtual shared memory. In cache based systems these techniques are very effective and lead to miss rates which are close to the minimum. In virtual shared memory systems, the techniques are also effective but leave room for additional improvements.

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