ECONO: Express coherence notifications for efficient cache coherency in many-core CMPs

Abstract

It is commonly stated that a directory-based coherence protocol is the design of choice to provide maximum performance in coherence maintenance for shared-memory many-core CMPs. Nevertheless, new solutions are emerging to achieve acceptable levels of on-chip area overhead and energy consumption to also meet scalability. In this work, we propose the Express COherence NOtification (ECONO) protocol, a coherence protocol aimed at providing high performance with minimal on-chip area and energy consumption for superior scalability. To maintain coherence, ECONO relies on express coherence notifications which are broadcast atomically over a dedicated lightweight and power-efficient on-chip network leveraging state-of-the-art technology. We implement and evaluate ECONO utilizing full-system simulation, a representative set of benchmarks, and compare it against two contemporary coherence protocols: Hammer and Directory. While ECONO achieves slightly better performance than Directory, our proposal does not need to encode sharer sets like in Hammer, saving significant on-chip area and energy even when considering the extra hardware resources required by ECONO. Projections for a 1024-core CMP reveal that, in comparison to one of the most scalable directory-based protocols to date, ECONO entails more than 2× less on-chip storage overhead while keeping with reasonable power dissipation.