A tagless coherence directory

Abstract

A key challenge in architecting a CMP with many cores is maintaining cache coherence in an efficient manner. Directory-based protocols avoid the bandwidth overhead of snoop-based protocols, and therefore scale to a large number of cores. Unfortunately, conventional directory structures incur significant area overheads in larger CMPs. The <i>Tagless Coherence Directory (TL)</i> is a scalable coherence solution that uses an implicit, conservative representation of sharing information. Conceptually, TL consists of a grid of small Bloom filters. The grid has one column per core and one row per cache set. TL uses 48% less area, 57% less leakage power, and 44% less dynamic energy than a conventional coherence directory for a 16-core CMP with 1MB private L2 caches. Simulations of commercial and scientific workloads indicate that TL has no statistically significant impact on performance, and incurs only a 2.5% increase in bandwidth utilization. Analytical modelling predicts that TL continues to scale well up to at least 1024 cores.

DOI: 10.1145/1669112.1669166

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