Modern-Day transistor technique enables the industry to integrate many cores on a single chip. As an increasing number of cores being integrated on a single chip, cache coherence has become an intractable issue as well as a bottleneck of performance. In this paper, the origin of cache coherence is carefully described. Further, the paper summarizes the key issue of cache coherence and reviews the study in this field a decade after entering the mulit-core era. From aspects of memory access, directory organization, coherence granularity, coherence traffic and scalability, the work on optimization of cache coherence in recent researches is also presented. Finally, the potential challenges in current coherence protocol and direction of future research are discussed.