Dealing with Traffic-Area Trade-Off in Direct Coherence Protocols for Many-Core CMPs

In many-core CMP architectures, the cache coherence protocol is a key component since it can add requirements of area and power consumption to the final design and, therefore, it could restrict severely its scalability. Area constraints limit the use of precise sharing codes to smallor medium-scale CMPs. Power constraints make impractical to use broadcast-based protocols for large-scale CMPs. Token-CMP and DiCo-CMP are cache coherence protocols that have been recently proposed to avoid the indirection problem of traditional directory-based protocols. However, Token-CMP is based on broadcasting requests to all tiles, while DiCo-CMP adds a precise sharing code to each cache entry. In this work, we address the traffic-area trade-off for these indirection-aware protocols. In particular, we propose and evaluate several implementations of DiCo-CMP which differ in the amount of coherence information that they must store. Our evaluation results show that our proposals entail a good traffic-area trade-off by halving the traffic requirements compared to Token-CMP and considerably reducing the area storage required by DiCo-CMP.