This paper presents the communication architecture of the START-VOYAGER system, a parallel machine composed of a cluster of unmodified IBM 604e-based SMP's connected via a high speed interconnection network. A custom network interface unit (NIU) plugs into a processor card slot of each SMP, providing a high-performance message passing substrate that supports both fast user-level message passing and cache-line coherent shared memory. The substrate consists of four hardware implemented message passing mechanisms to achieve high performance over a wide spectrum of communication patterns. START-VOYAGER also introduces a novel protection scheme which improves upon past designs by not requiring strictly synchronized gang-scheduling and by allowing system code and multiple user applications to share the network simultaneously without compromising protection nor performance. Performance predictions based on synthesized Verilog code show START-VOYAGER's novel message passing mechanisms offer a definitive advantage in a multi-threaded environment without compromising the performance in a single-threaded environment. Preliminary shared memory simulations are also promising.
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