Token tenure: PATCHing token counting using directory-based cache coherence

Traditional coherence protocols present a set of difficult tradeoffs: the reliance of snoopy protocols on broadcast and ordered interconnects limits their scalability, while directory protocols incur a performance penalty on sharing misses due to indirection. This work introduces PATCH (Predictive/Adaptive Token Counting Hybrid), a coherence protocol that provides the scalability of directory protocols while opportunistically sending direct requests to reduce sharing latency. PATCH extends a standard directory protocol to track tokens and use token counting rules for enforcing coherence permissions. Token counting allows PATCH to support direct requests on an unordered interconnect, while a mechanism called token tenure uses local processor timeouts and the directory’s per-block point of ordering at the home node to guarantee forward progress without relying on broadcast. PATCH makes three main contributions. First, PATCH introduces token tenure, which provides broadcast-free forwa...

• Type: Conference
• Year: 2008
• Where: MICRO
• Authors: Arun Raghavan, Colin Blundell, Milo M. K. Martin
• Keywords: Coherence Protocol, Directory Protocols, Hardware, MICRO 2008, Token Counting