Racer: TSO consistency via race detection
Alberto Ros, Stefanos Kaxiras · 2016
Several recent efforts aim to simplify coherence and its associate costs (e.g., directory size, complexity) in multicores. The bulk of these efforts rely on program data-race-free (DRF) semantics to eliminate explicit invalidations and use self-invalidation instead. While such protocols are simple, they require software cooperation. This is acceptable only for (correct) software that abides by the SC-for-DRF semantics defined in many modern programming language standards (e.g., C++11, Java, the latest C standards) but many are unwilling to trust coherence that relies solely on program semantics for its correctness. To address this important issue, this work proposes Racer, an efficient self-invalidation/write-through approach that guarantees the memory consistency model of the most common family of processors (TSO-x86), and at the same time maintains the relaxed-ordering advantages of SC-for-DRF protocols. Lacking a directory and explicit invalidations, Racer achieves this by detecting read-after-write races and causing self-invalidation on the racing reader's cache. Racer also uses a coalescing store buffer (at the L1 level) that allows coalescing and reordering of stores but upon detecting a race, delays the racing read until all its stores appear in order to the read. Race detection is performed using an efficient signature-based mechanism at the level of the shared cache. Racer performs significantly better than a traditional non-scalable directory-based protocol that does not allow reordering at the protocol level (14.2% in time and 26.4% in energy), a directory protocol for TSO (1.9% in time and 15.5% in energy), and state-of-the-art SC-for-DRF protocol that relies on acquire-release annotations in the programs (6.7% in time and 9.5% in energy). Racer self-invalidates less than program-level annotations as it only enforces ordering on dynamically detected races and provides significant reductions in network traffic and memory-system energy consumption.