On the use of coscheduling in fast communication systems

Coscheduling of communication and computation is considered one of the crucial points to obtain good performance out of fast communication systems. Various techniques have been examined in literature ranging from strict "gang scheduling" of all processes possibly involved in message exchange to "implicit coscheduling" in which the communication support system may act on the scheduling of sending and receiving processes trying to improve performance without explicit coordination by special purpose message exchanges. Based on the experience in implementing the GAMMA communication system, we are convinced that some form of coscheduling is needed in order to obtain best performance in communication. However we believe that most of the approaches described in literature so far are too simplistic to be really effective. In this paper we point out and classify some of the major problems a system that attempts to coschedule communication and computation should address. We hope to clarify the goals of a coscheduler by taking some of the crucial characteristics of the communication into account. We also hope to be able to devise some more integrated and coherent strategies of coordination between process scheduling and choice of communication modes.