HMG: Extending Cache Coherence Protocols Across Modern Hierarchical Multi-GPU Systems

Prior work on GPU cache coherence has shown that simple hardware-or software-based protocols can be more than sufficient. However, in recent years, features such as multi-chip modules have added deeper hierarchy and non-uniformity into GPU memory systems. GPU programming models have chosen to expose this non-uniformity directly to the end user through scoped memory consistency models. As a result, there is room to improve upon earlier coherence protocols that were designed only for flat single-GPU hierarchies and/or simpler memory consistency models. In this paper, we propose HMG, a cache coherence protocol designed for forward-looking multi-GPU systems. HMG strikes a balance between simplicity and performance: it uses a readily-implementable VI-like protocol to track coherence states, but it tracks sharers using a hierarchical scheme optimized for mitigating the bandwidth limitations of inter-GPU links. HMG leverages the novel scoped, non-multi-copy-atomic properties of modern GPU memory models, and it avoids the overheads of invalidation acknowledgments and transient states that were needed to support prior GPU memory models. On a 4-GPU system, HMG improves performance over a software-controlled, bulk invalidation-based coherence mechanism by 26% and over a non-hierarchical hardware cache coherence protocol by 18%, thereby achieving 97% of the performance of an idealized caching system.