Efficient Inspected Critical Sections in Data-Parallel GPU Codes

Optimistic concurrency control and STMs rely on the assumption of sparse conflicts. For data-parallel GPU codes with many or with dynamic data dependences, a pessimistic and lock-based approach may be faster, if only GPUs would offer hardware support for GPU-wide fine-grained synchronization. Instead, current GPUs inflict dead- and livelocks on attempts to implement such synchronization in software. The paper demonstrates how to build GPU-wide non-hanging critical sections that are as easy to use as STMs but also get close to the performance of traditional fine-grained locks. Instead of sequentializing all threads that enter a critical section, the novel programmer-guided Inspected Critical Sections (ICS) keep the degree of parallelism up. As in optimistic approaches threads that are known not to interfere, may execute the body of the inspected critical section concurrently.