OCTET

Abstract

Parallel programming is essential for reaping the benefits of parallel hardware, but it is notoriously difficult to develop and debug reliable, scalable software systems. One key challenge is that modern languages and systems provide poor support for ensuring concurrency correctness properties - atomicity, sequential consistency, and multithreaded determinism - because all existing approaches are impractical. Dynamic, software-based approaches slow programs by up to an order of magnitude because capturing and controlling cross-thread dependences (i.e., conflicting accesses to shared memory) requires synchronization at virtually every access to potentially shared memory. This paper introduces a new software-based concurrency control mechanism called OCTET that soundly captures cross-thread dependences and can be used to build dynamic analyses for concurrency correctness. OCTET achieves low overheads by tracking the locality state of each potentially shared object. Non-conflicting accesses conform to the locality state and require no synchronization; only conflicting accesses require a state change and heavyweight synchronization. This optimistic tradeoff leads to significant efficiency gains in capturing cross-thread dependences: a prototype implementation of OCTET in a high-performance Java virtual machine slows real-world concurrent programs by only 26% on average. A dependence recorder, suitable for record & replay, built on top of OCTET adds an additional 5% overhead on average. These results suggest that OCTET can provide a foundation for developing low-overhead analyses that check and enforce concurrency correctness.