Predictable Cache Coherence for Multi-Core Real-Time Systems
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This work addresses the challenge of allowing simultaneous and predictable accesses to shared data on multi-core systems. We accomplish this by proposing a predictable cache coherence protocol, which mandates the use of certain invariants to ensure predictability. In particular, we enforce these invariants by augmenting the classic modify-share-invalid (MSI) protocol with transient coherence states, and minimal architectural changes. This allows us to derive worst-case latency bounds on predictable MSI (PMSI) protocol. Our analysis shows that while the arbitration latency scales linearly, the coherence latency scales quadratically with the number of cores. We implement PMSI in gem5, and execute SPLASH-2 and synthetic multi-threaded workloads. Our empirical results show that our approach is always within the analytical worst-case latency bounds, and that PMSI improves average-case performance by up to 4x over the next best predictable alternative. PMSI has average slowdowns of 1.45x and 1.46x compared to conventional MSI and MESI protocols, respectively.
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