The directory-based cache coherence protocol for the DASH multiprocessor

Abstract

DASH is a scalable shared-memory multiprocessor currently being developed at Stanford's Computer Systems Laboratory. The 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 directory-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. In this paper, we present the design of the DASH coherence protocol and discuss how it addresses the above issues. We also discuss our strategy for verifying the correctness of the protocol and briefly compare our protocol to the IEEE Scalable Coherent Interface protocol.