Multicast snooping: a new coherence method using a multicast address network

  • Authors:

  • E. Ender Bilir;Ross M. Dickson;Ying Hu;Manoj Plakal;Daniel J. Sorin;Mark D. Hill;David A. Wood

  • Affiliations:

  • Computer Sciences Department, University of Wisconsin-Madison;Computer Sciences Department, University of Wisconsin-Madison;Computer Sciences Department, University of Wisconsin-Madison;Computer Sciences Department, University of Wisconsin-Madison;Computer Sciences Department, University of Wisconsin-Madison;Computer Sciences Department, University of Wisconsin-Madison;Computer Sciences Department, University of Wisconsin-Madison

  • Venue:
  • ISCA '99 Proceedings of the 26th annual international symposium on Computer architecture
  • Year:
  • 1999

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Abstract

This paper proposes a new coherence method called "multicast snooping" that dynamically adapts between broadcast snooping and a directory protocol. Multicast snooping is unique because processors predict which caches should snoop each coherence transaction by specifying a multicast "mask." Transactions are delivered with an ordered multicast network, such as an Isotach network, which eliminates the need for acknowledgment messages. Processors handle transactions as they would with a snooping protocol, while a simplified directory operates in parallel to check masks and gracefully handle incorrect ones (e.g., previous owner missing). Preliminary performance numbers with mostly SPLASH-2 benchmarks running on 32 processors show that we can limit multicasts to an average of 2-6 destinations (<< 32) and we can deliver 2-5 multicasts per network cycle (>> broadcast snooping's 1 per cycle). While these results do not include timing, they do provide encouragement that multicast snooping can obtain data directly (like broadcast snooping) but apply to larger systems (like directories).