WAYPOINT: scaling coherence to thousand-core architectures

  • Authors:
  • John H. Kelm;Matthew R. Johnson;Steven S. Lumettta;Sanjay J. Patel

  • Affiliations:
  • University of Illinois, Urbana, IL, USA;University of Illinois, Urbana, IL, USA;University of Illinois, Urbana, IL, USA;University of Illinois, Urbana, IL, USA

  • Venue:
  • Proceedings of the 19th international conference on Parallel architectures and compilation techniques
  • Year:
  • 2010

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Abstract

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.