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In this paper, we present the design, implementation and evaluation of a new distributed shared memory (DSM) coherence model called multiple-writer entry consistency (MEC). MEC combines the efficient communication mechanisms of Lazy Release Consistency (LRC) with the flexible data management of the Shared Regions and Entry Consistency (EC) models. This is achieved in MEC by decoupling synchronization from coherence (in contrast to the tight coupling of synchronization and coherence present in EC) while retaining the familiar synchronization structure found in Release Consistent (RC) programs. The advantage of MEC is that it allows region-based coherence protocols (those that manage data at the granularity of user-defined shared regions) to be used along side page-based protocols within an application and within the RC framework. Our experimental evaluation on an 8 processor system shows that using MEC reduces parallel execution times by margins ranging from 5% to 46% in five of the six applications that we study. However, the parallel execution time of the LRC version of the remaining application is lower than the MEC version by 48%. We conclude that offering both page-based and region-based models for coherence within the same system is not only practical but necessary.
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