I n 1994, we presented extensions to M P I and offered an early paper on potential thread extensions to MPI, as well as non-blocking collective extensions to MPI [ld]. The present paper is a thorough review of thread issues in MPI, including alternative models, their computational uses, and the impact on implementations. A number of issues are addressed: barriers to thread safety in MPI implementations with MPICH as an example and changes of the semantics of non-thread-safe MPI calls, different thread models, their uses, and possible integration. Minimal portable thread management and synchronization mechanisms A P I extensions for MPI are considered. A tentative design for multi-threaded thread-safe A D I and Channel Device for MPICH is proposed. W e consider threads as both an implementation device for M P I and as a user-level mechanism to achieve fine-grain concurrency. The reduction of the process to a simple resource container (as considered by Mach), with the thread as the main named computational unit is suggested. Specific results thus far with Windows N T version of MPICH are mentioned.
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