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Active Messages

The algorithms we developed require active message remote procedure calls in order to minimize communications overhead and obtain good relative speedup. Active message remote procedure calls (RPCs) provide protocol-less access to the transport layer of the communication network on the CM-5. The user must assume responsibilities for all aspects of communications --- as a result, active message RPCs provide very low-latency communications. Active messages provide extremely fast interprocessor communications, and also permit a parallel-code development paradigm that greatly simplifies the implementation of these sparse matrix algorithms. Empirical data has been collected on both active message-based implementations and more traditional cooperative buffer-based message passing commands in order to illustrate the need for low latency communications when solving matrices that are as sparse and irregular as power systems matrices.

Active messages are an example of a distributed-memory multiprocessor message-passing paradigm that has been developed by the supercomputer community to provide extremely low latency communications. All other CM-5 message-passing software is built upon this low latency protocol-less remote procedure call. The empirical data presented in this paper clearly shows that low-latency communications are required for direct linear solvers for power systems network matrices. This research has been performed on a small version (only 32 processors) of a massively parallel processing (MPP) architecture, the Thinking Machines CM-5. We believe that scalable parallel processing (SPP) architectures, like the IBM SP2, may eventually provide similar low-latency communications for short messages, in addition to expanded network bandwidth, because there are many parallel algorithms that can only be implemented efficiently with this type of interprocessor communications support. SPP hardware developers recognize that low-latency communications increase the utility of their computer and, consequently, improve market potential.



David P. Koester
Sun Oct 22 15:31:10 EDT 1995