Switch-Based Packing Technique for Improving Token Coherence Scalability

Abstract

Traditional cache coherence protocols either provide low latency cache misses (snooping protocols) or bandwidth efficiency (directory protocols). To simultaneously capture the best attributes of traditional protocols, Token Coherence has been recently proposed. This protocol can quickly resolve cache misses by transient requests. However, since transient requests are unordered messages, they may sometimes fail in solving cache misses mainly due to the occurrence of protocol races. Thus, when the completion of cache misses is not possible by transient requests, Token Coherence uses a starvation prevention mechanism to ensure their completion. Although several implementation options of starvation prevention mechanisms have been proposed, all of them are broadcast-based. This fact represents a large detriment to the Token Coherence scalability. To tackle this problem, in this work we apply a switch-based packing technique that alleviates the harm of broadcast messages and improves the protocol scalability.