SCsafe: Logging sequential consistency violations continuously and precisely

Abstract

Sequential Consistency Violations (SCV) in relaxed consistency machines cause programs to malfunction and are hard to debug. While there are proposals for detecting and recording SCVs, they are limited in that they end program execution after detecting the first SCV because the program is now non-SC. Therefore, they cannot be used in production runs. In addition, such proposals rely on complicated hardware. To address these problems, this paper proposes the first architecture that detects and logs SCVs in a continuous manner, while retaining SC. In addition, the scheme is precise and uses substantially simpler hardware. The scheme, called SCsafe, operates continously because, after SCV detection and logging, it recovers and resumes execution while retaining SC. As a result, it can be used in production runs. In addition, SCsafe is precise in that it identifies only true SCVs - rather than dependence cycles due to false sharing. Finally, SCsafe's hardware is mostly local to each processor, and uses known recovery techniques. We evaluate SCsafe using simulations of 16-processor multicores with Total Store Order or Release Consistency. In codes with SCVs, SCsafe detects and reports SCVs while enforcing SC during the execution. In codes with few SCVs, it adds a negligible performance overhead. Finally, SCsafe is scalable with the processor count.