Snooping and Ordering Ring - An Efficient Cache Coherence Protocol for Ring Connected CMP

Ring is a promising on-chip interconnection for CMP. It is more scalable than bus and much simpler than packet-switched networks. The ordering property of ring can be used to optimize cache coherence protocol design. Existing ring protocols, such as the snooping ring protocol and the ring-order protocol need a retry and acknowledgement scheme or use the ordering property of the ring respectively to resolve conflict memory requests. A cache coherence protocol named SOR (Snooping and Ordering Ring) is developed for ring connected CMP in this paper. This protocol is based on the snooping ring protocol. But instead of using the acknowledgement and retry scheme, it uses the ordering property of the ring to resolve conflicts, thus can avoid unnecessary retries to improve performance and power efficiency. The L1 snooping results are sent with the requests instead of being delayed, so that many useless snoops can be avoided. Simulation result shows that the average probe slot transports and snoop operations can be reduced by SOR are 47% and 48.9%. The average and maximum performance improvements by SOR are 3.33% and 6%.