Fast Collective Operations Using Shared and Remote Memory Access Protocols on Clusters

This paper describes a novel methodology for implementing a common set of collective communication operations on clusters based on symmetric multiprocessor (SMP) nodes. Called Shared-Remote-Memory collectives, or SRM, our approach replaces the point-to-point message passing, traditionally used in implementation of collective message-passing operations, with a combination of shared and remote memory access (RMA) protocols that are used to implement semantics of the collective operations directly. Appropriate embedding of the communication graphs in a cluster maximizes the use of shared memory and reduces network communication. Substantial performance improvements are achieved over the highly optimized commercial IBM implementation and the open-source MPICH implementation of MPI across a wide range of message sizes on the IBM SP. For example, depending on the message size and number of processors, SRM implementation of broadcast, reduce, and barrier outperforms IBM MPI_Bcast by 27-84%, MPI_Reduce by 24- 79%, and MPI_Barrier by 73 % on 256 processors, respectively. 1