WAYPOINT: scaling coherence to thousand-core architectures

John H. Kelm, Matthew R. Johnson, Steven S. Lumettta, Sanjay J. Patel
University of Illinois at Urbana-Champaign, Urbana, IL 61801
Proceedings of the 19th international conference on Parallel architectures and compilation techniques, PACT ’10, 2010


   title={WAYPOINT: scaling coherence to thousand-core architectures},

   author={Kelm, J.H. and Johnson, M.R. and Lumettta, S.S. and Patel, S.J.},

   booktitle={Proceedings of the 19th international conference on Parallel architectures and compilation techniques},





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In this paper, we evaluate a set of coherence architectures in the context of a 1024-core chip multiprocessor (CMP) tailored to throughput-oriented parallel workloads. Based on our analysis, we develop and evaluate two techniques for scaling coherence to thousand-core CMPs. We find that a broadcast-based probe filtering scheme provides reasonable performance up to 128 cores for some benchmarks, but is not generally scalable. We propose a broadcast-collective network for accelerating probe filter misses, which extends scalability but falls short of supporting 1024 cores. We find that a sparse directory with an invalidate-on-evict policy can work well for many throughput-oriented workloads. However, the on-die structures required to achieve good performance carry a large performance and power overhead. To achieve thousand-core scalability with smaller and less associative sparse directories, we introduce WayPoint, a mechanism that increases directory associativity and capacity dynamically. Using less than 3% of total die area, Way-Point achieves performance within 4% of an infinitely large on-die directory.
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