Designing scalable and efficient Message Passing Interface (MPI) implementations for emerging cluster interconnects such as VIA-based networks and InfiniBand are important building next generation clusters. In this paper, we address the scalability issue in the implementation of MPI over VIA by on-demand connection management mechanism. The on-demand connection management is designed to limit the use of resources to what applications absolutely require. We address the design issues of

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... ing the ondemand connection mechanism into an implementation of MPI over VIA. A complete implementation was done for MVICH over both cLAN VIA and Berkeley VIA. Performance evaluation on a set of microbenchmarks and NAS parallel benchmarks demonstrates that the on-demand mechanism can increase the scalability of MPI implementations by limiting the use of resources as needed by applications. It also shows that the on-demand mechanism delivers comparable or better performance as the static mechanism in which a fully-connected process model usually exists in the MPI implementations. These results demonstrate that the on-demand connection mechanism is a feasible solution to increase the scalability of MPI implementations over VIA-and InfiniBand-based networks.