Hardware Support for Flexible Distributed Shared Memory

Workstation-based parallel systems are attractive due to their low cost and competitive uniprocessor performance. However, supporting a cache-coherent global address space on these systems involves significant overheads. We examine two approaches to coping with these overheads. First, DSM-specific hardware can be added to the off the-shelf component base to reduce overheads. Second, application-specific coherence protocols can avoid some overheads by exploiting programmer (or compiler) knowledge of an application's communication patterns. To explore the interaction between these approaches, we simulated four designs that add DSM acceleration hardware to a collection of off-the-shelf workstation nodes. Three of the designs support user-level software coherence protocols, enabling application-specific protocol optimizations. To verify the feasibility of our hardware approach, we constructed a prototype of the simplest design. Measured speedups from the prototype match simulation results closely. We find that, even with aggressive DSM hardware support, custom protocols can provide significant speedups for some applications. In addition, the custom protocols are generally effective at reducing the impact of other overheads, including those due to less aggressive hardware support and larger network latencies. However, for three of our benchmarks, the additional hardware acceleration provided by our most aggressive design avoid the need to develop more efficient custom protocols.