Implementing Split-Mode Barriers in OpenSHMEM

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Barriers synchronize the state of many processing elements working in parallel. No worker may leave a barrier before all the others have arrived. High performance applications hide latency by keeping a large number of operations in progress asynchronously. Since barriers synchronize all these operations, maximum performance requires that barriers have as little overhead as possible. When some workers arrive at a barrier much later than others, the early arrivers must sit idle waiting for them. Split-mode barriers provide barrier semantics while also allowing the early arrivers to make progress on other tasks. In this paper we describe the process and several challenges in developing split-mode barriers in the OpenSHMEM programming environment.