Clusters of multiprocessors, or Clumps, promise to be the supercomputers of the future, but obtaining high performance on these architectures requires an understanding of interactions between the multiple levels of interconnection. In this paper, we present the first multi-protocol implementation of a lightweight message layer---a version of Active Messages-II running on a cluster of Sun Enterprise 5000 servers connected with Myrinet. This research brings together several pieces of high-performance interconnection technology: bus backplanes for symmetric multiprocessors, low-latency networks for connections between machines, and simple, user-level primitives for communication. The paper describes the shared memory message-passing protocol and analyzes the multi-protocol implementation with both microbenchmarks and Split-C applications. Three aspects of the communication layer are critical to performance: the overhead of cache-coherence mechanisms, the method of managing concurrent access, and thecost of accessing state with the slower protocol. Through the use of an adaptive polling strategy, the multi-protocol implementation limits performance interactions between the protocols, delivering up to 160 MB/s of bandwidth with 3.6 microsecond end-to-end latency. Applications within an SMP benefit from this fast communication, running up to 75% faster than on a network of uniprocessor workstations. Applications running on the entire Clump are limited by the balance of NIC's to processors in our system, and are typically slower than on the NOW. These results illustrate several potential pitfalls for the Clumps architecture.
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