Multi-grain coherence directories

Conventional directory coherence operates at the finest granularity possible, that of a cache block. While simple, this organization fails to exploit frequent application behavior: at any given point in time, large, continuous chunks of memory are often accessed only by a single core. We take advantage of this behavior and investigate reducing the coherence directory size by tracking coherence at multiple different granularities. We show that such a Multi-grain Directory (MGD) can significantly reduce the required number of directory entries across a variety of different workloads. Our analysis shows a simple dual-grain directory (DGD) obtains the majority of the benefit while tracking individual cache blocks and coarse-grain regions of 1KB to 8KB. We propose a practical DGD design that is transparent to software, requires no changes to the coherence protocol, and has no unnecessary bandwidth overhead. This design can reduce the coherence directory size by 41% to 66% with no statistically significant performance loss.