Mobility

SCD: A Scalable Coherence Directory With Flexible Sharer Set Encoding

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Executive Summary

Large-scale CMPs with hundreds of cores require a directory-based protocol to maintain cache coherence. However, previously proposed coherence directories are hard to scale beyond tens of cores, requiring either excessive area or energy, complex hierarchical protocols, or inexact representations of sharer sets that increase coherence traffic and degrade performance. The authors present SCD, a scalable coherence directory that relies on efficient highly-associative caches (such as z-caches) to implement a single-level directory that scales to thousands of cores, tracks sharer sets exactly, and incurs negligible directory-induced invalidations.

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