Decoupled Hardware Support for Distributed Shared Memory

This paper investigates hardware support for fine-grain distributed shared memory (DSM) in networks of workstations. To reduce design time and implementation cost relative to dedicated DSM systems, we decouple the functional hardware components of DSM support, allowing greater use of off-the-shelf devices.We present two decoupled systems, Typhoon-0 and Typhoon-1. Typhoon-0 uses an off-the-shelf protocol processor and network interface; a custom access control device is the only DSM-specific hardware. To demonstrate the feasibility and simplicity of this access control device, we designed and built an FPGA-based version in under one year. Typhoon-1 also uses an off-the-shelf protocol processor, but integrates the network interface and access control devices for higher performance.We compare the performance of the two decoupled systems with two integrated systems via simulation. For six benchmarks on 32 nodes, Typhoon-0 ranges from 30% to 309% slower than the best integrated system, while Typhoon-1 ranges from 13% to 132% slower. Four of the six benchmarks achieve speedups of 12 to 18 on Typhoon-0 and 15 to 26 on Typhoon-1, compared with 19 to 35 on the best integrated system. Two benchmarks are hampered by high communication overheads, but selectively replacing shared-memory operations with message passing provides speedups of at least 16 on both decoupled systems. These speedups indicate that decoupled designs can potentially provide a cost-effective alternative to complex high-end DSM systems.