The case for a scalable coherence protocol for complex on-chip cache hierarchies in many core systems

This paper introduces a new coherence protocol that addresses the challenges of complex multilevel cache hierarchies in future many-core systems. In order to keep coherence protocol complexity bounded, inclusiveness is required to track coherence information across levels in this type of systems, but this might introduce unsustainable costs for directory structures. Cost reduction decisions taken to reduce this complexity may introduce artificial inefficiencies in the on-chip cache hierarchy, especially when the number of cores and private caches size is large. The coherence protocol presented in this work, denoted MOSAIC, introduces a new approach to tackle this problem. In energy terms, the protocol scales like a conventional directory coherence protocol, but relaxes the shared information inclusiveness. This allows the performance implications of directory size and associativity reduction to be overcome. Contrary to the common belief that inclusiveness is inescapable when attempting to maintain complexity constrained, MOSAIC is even simpler than a conventional directory. The results of our evaluation show that the approach is quite insensitive, in terms of performance and energy expenditure, to the size and associativity of the directory.