The directory-based cache coherence protocol for the DASH multiprocessor

DASH is a scalable shared-memory multiprocessor whose architecture consists of powerful processing nodes, each with a portion of the shared-memory, connected to a scalable interconnection network. A key feature of DASH is its distributed direction-based cache coherence protocol. Unlike traditional snoopy coherence protocols, the DASH protocol does not rely on broadcast; instead it uses point-to-point messages sent between the processors and memories to keep caches consistent. Furthermore, the DASH system does not contain any single serialization or control point. While these features provide the basis for scalability, they also force a reevaluation of many fundamental issues involved in the design of a protocol. These include the issues of correctness, performance, and protocol complexity. The design of the DASH coherence protocol is presented and discussed from the viewpoint of how it addresses the above issues. Also discussed is a strategy for verifying the correctness of the protocol. A brief comparison of the protocol with the IEEE Scalable Coherent Interface protocol is made