Manager-client pairing: A framework for implementing coherence hierarchies

As technology continues to scale, the need for more sophisticated coherence management is becoming a necessity. The likely solution to this problem is the use of coherence hierarchies, analogous to how cache hierarchies have helped address the memory-wall problem in the past. Previous work in the construction of large-scale coherence protocols, however, demonstrates the complexity inherent to this design space. The difficulty with hierarchical coherence protocol design is the complexity increases exponentially with the increase in coherence states, due in turn to interactions between hierarchy tiers. Additionally, because of the large development investment, choices regarding coherence hierarchy are often made statically with little knowledge of how changes to the organization would affect the system. In this work, we present Manager-Client Pairing (MCP) as a unifying methodology for designing multi-tier coherence protocols by formally defining and limiting the interactions between levels within a coherence hierarchy to enable composition. Using MCP, we then implement a variety of hierarchical coherence protocol configurations for a 256-core system comprised of 4 64-core manycores, and provide insights into what impact different hierarchy depth and width choices can have on system performance.