A NoC-level Support for Broadcast-based Coherence Protocols

Chip Multiprocessor Systems (CMPs) rely on a cache coherency protocol to maintain memory access coherence between cached data and main memory. The Hammer coherency protocol is appealing as it eliminates most of the space overhead when compared to a directory protocol. However, it generates much more traffic, thus stressing the NoC and having worse performance in terms of power consumption. When using a NoC with built-in broadcast support network utilization is lowered but does not solve completely the problem as acknowledgment messages are still sent from each core to the memory access requestor. In this paper we propose a small and simple control network that collects the acknowledgement messages and delivers them with a bounded and fixed latency, thus relieving the NoC from a large amount of messages. Experimental results demonstrate on a 16-tile system with the control network that execution time improves up to 17%, with an average improvement of about 7.5%. Keywords—CMPs, cache hierarchy, NoC