Chip multiprocessors (CMPs) with on-chip network connecting processor cores have been pervasively accepted as a promising technology to efficiently utilize the ever increasing density of transistors on a chip. Communications in CMPs require invalidating cached copies of a shared data block. The coherence traffic incurs more and more significant overhead as the number of cores in a CMP increases. Conventional designs of cache coherence protocols do not take into account characteristics of underlying networks for flexibility reasons. However, in CMPs, processor cores and the on-chip network are tightly integrated. Exposing the network features to cache coherence protocols will unveil some optimization opportunities. In this paper, we propose distance aware protocol and multi-target invalidations, which exploit the network characteristics to reduce the invalidation traffic overhead at negligible hardware cost. Experimental results on a 16-core CMP simulator showed that the two mechanisms reduced the average invalidation traffic latency by 5%, up to 8%.
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