Reducing the Read-miss Penalty for Flat COMA Protocols

Abstract

In flat cache-only memory architectures (COMA), an attraction-memory miss must first interrogate a directory before a copy of the requested data can be located, which often involves three network traversals. By keeping track of the identity of a potential holder of the copy-called a hint-one network traversal can be saved which reduces the read penalty. We have evaluated the reduction of the read-miss penalty provided by hints using detailed architectural simulations and four benchmark applications. The results show that a previously proposed protocol using hints can actually make the read-miss penalty larger because when the hint is not correct, an extra network traversal is needed. This has motivated us to study a new protocol, using hints, that simultaneously sends a request to the potential holder and to the directory. This protocol reduces the read-miss penalty for all applications but the protocol complexity does not seem to justify the performance improvement.