Broadcast directory: A scalable cache coherent architecture for mesh-connected multiprocessors

Abstract

Abstract Large-scale shared memory multiprocessors favor a directory-based cache coherence scheme for its scalability. The directory space needed to record the information for sharers has a complexity of Θ(N2) when a full-mapped vector is used for an N-node system. Although this overhead can be reduced by limiting the directory size assuming that the sharing degree is small, it will experience significant inefficiency when data is widely shared. In this paper, we propose a new directory scheme and a cache coherence scheme based on it for a mesh interconnection. Deterministic and wormhole routing enables a pointer to represent a set of nodes. Also a message traversing on the mesh performs a broadcast mission to a set of nodes without extra traffic, which can be utilized for the cache coherence protocol. Only a slight change on a typical router is needed to implement our scheme. This scheme is also applicable to any k-ary n-cube networks including a mesh. The Splash-2 parallel program suite is used in the simulation study, where our scheme is compared with other directory-based schemes. Our scheme is proved to generate much less traffic for cache coherence while the space complexity is more scalable Θ(N 3/2 log N) .