The Semantics of Blocking and Nonblocking Send and Receive Primitives

Current message-passing parallel computers provide send and receive primitives with a wide variety of blocking, synchronization , selectivity and ordering properties. Unfortunately , the interactions between the diierent properties of the send and receive primitives can be extremely complex, and as a result, the precise semantics of these primitives are not well understood. In this paper we present formal models for message-passing systems that provide both synchronous and asynchronous sends, both blocking and nonblocking sends and receives, and a variety of ordering properties. In addition, the receive primitives are very general in that they can specify the desired source and/or tag value of a message. Our models apply to all message-passing programs, including ones with errors, and they apply to parallel computers with arbitrary amounts of buuering. To the best of our knowledge , this is the rst time that such rich message-passing models have been deened formally. In addition to presenting the formal models, we also give a number of theorems that capture the properties of the models.