Region-level Tracking for Scalable Directory Cache

Abstract

Traditional coherence directories track sharing information at a cache-line granularity. In practice, however, as data sharing occurs at a coarser granularity in a large region of memory, common sharing patterns tend to be observed across multiple proximate lines. Hence, the directory entries for the lines replicate the same sharing information, resulting in inefficient use of space, power, and energy. In this paper, we empirically demonstrate "region-level sharing pattern locality", that is, a small number of distinct sharing patterns are observed across proximate lines within a large region of memory, e.g., a page unit. We leverage this phenomenon to propose a new representation of sharing information, called Region-level Sharing information Tracking (RST), that dynamically maintains common sharing information in a space-efficient manner at a region-level. Our experimental results based on conventional parallel and server workloads show that RST reduces over 75% of the area (and hence energy) compared to conventional directory caches, with almost negligible performance overhead.