Evaluating the Effect of Coherence Protocols on the Performance of Parallel Programming Constructs1

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The different implementations of parallel programming constructs interact heavily with a multiprocessor's coherence protocol and thus may have a significant impact on performance. The form and extent of this interaction have not been established so far however, particularly in the case of update-based coherence protocols. In this paper we study the running time and communication behavior of ticket and MCS spin locks; centralized, dissemination, and tree-based barriers; parallel and sequential reductions; linear broadcasting and producer and consumer-driven logarithmic broadcasting; and centralized and distributed task queues, under pure and competitive update coherence protocols on a scalable multiprocessor; results for a write invalidate protocol are presented mostly for comparison purposes. Our experiments indicate that parallel programming techniques that are well-established for write invalidate protocols, such as MCS locks and task queues, are often inappropriate for update-based protocols. In contrast, techniques such as dissemination and tree barriers achieve superior performance under update-based protocols. Our results also show that update-based protocols sometimes lead to different design decisions than write invalidate protocols. Our main conclusion is that indeed the interaction of the parallel programming constructs with the multiprocessor's coherence protocol has a significant impact on performance. The implementation of these constructs must be carefully matched to the coherence protocol if ideal performance is to be achieved.

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