MESIF: A Two-Hop Cache Coherency Protocol for Point-to-Point Interconnects (2009)

Abstract

We describe MESIF, the first source-snooping cache coherence protocol. Built on point-to-point communication links, the protocol maintains no directory, and mimics the broadcast behavior of a snooping cache protocol. MESIF supplies data cached in other nodes in a single round-trip delay (2-hop latency) for all common operations. Because of the speed of the links, the protocol can outperform a bus-based protocol for a small number of nodes, but scales through hierarchical extension to a large-scale multiprocessor system. Salient features of the protocol are described. The introduction of a novel forwarding state is used to assure a single response to shared data and to simplify conflict resolution. In the hierarchical extension, auxiliary hardware data structures can be included to provide 2-hop latency for most operations. The recently revealed Intel® Quick-Path InterconnectTM protocol is derived from MESIF. Some design differences are highlighted.