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Design and Performance of SMPs with Asynchronous Caches
Pong, Fong; Dubois, Michel; Lee, Ken
Keyword(s): asynchronous; cache coherence; shared-memory multiprocessor; SMP
Abstract: We propose and evaluate a cache-coherent symmetric multiprocessor system (SMP) based on asynchronous caches. In a system with asynchronous caches, processors and memory controllers may observe the same coherence request at different points in time. All protocol transactions are uni-directional and processors do not report snoop results. The need for an extensive interlocking protocol between processor nodes and memory controller which is characteristic of snooping buses is thus removed. This design overcomes some of the scalability problem of a multi-drop shared-bus by employing high-speed point-to-point links, whose scalability prospects are much better than for shared buses. Memory and processors communicate through a set of queues. This highly pipelined memory system design is a better match to emerging ILP processors than bus-based snooping. Simulation results for ILP processors show that the shared-bus design is limited by its bandwidth. By contrast the parallel link design has ample bandwidth and yields large performance gain for the transaction processing and scientific benchmarks that we have considered. Besides higher performance the asynchronous design we propose considerably simplifies the behavior expected from the hardware. This is important because snooping bus protocols are so complex today that their verification has become a major challenge.
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