Multicast snooping: A new coherence method using a multicast address network
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).
