Photonic-based express coherence notifications for many-core CMPs

Directory-based coherence protocols (Directory) are considered the design of choice to provide maximum performance in coherence maintenance for shared-memory many-core CMPs, despite their large memory overhead. New solutions are emerging to achieve acceptable levels of on-chip area overhead and energy consumption such as optimized encoding of block sharers in Directory (e.g., SCD) or broadcast-based coherence (e.g., Hammer). In this work, we propose a novel and efficient solution for the cache coherence problem in many-core systems based on the co-design of the coherence protocol and the interconnection network. Particularly, we propose ECONO, a cache coherence protocol tailored to future many-core systems that resorts on PhotoBNoC, a special lightweight dedicated silicon-photonic subnetwork for efficient delivery of the atomic broadcast coherence messages used by the protocol. Considering a simulated 256-core system, as compared with Hammer, we demonstrate that ECONO+PhotoBNoC reduces performance and energy consumption by an average of 34% and 32%, respectively. Additionally, our proposal lowers the area overhead entailed by SCD by 2×.