Warp Scheduling for Fine-Grained Synchronization

Abstract

Fine-grained synchronization is employed in many parallel algorithms and is often implemented using busy-wait synchronization (e.g., spin locks). However, busy-wait synchronization incurs significant overheads and existing CPU solutions do not readily translate to single-instruction, multiple-thread (SIMT) graphics processor unit (GPU) architectures. In this paper, we propose Back-Off Warp Spinning (BOWS), a hardware warp scheduling policy that extends existing warp scheduling policies to temporarily deprioritize warps executing busy wait code. In addition, we propose Dynamic Detection of Spinning (DDOS), a novel hardware mechanism for accurately and efficiently detecting busy-wait synchronization on GPUs. On a set of GPU kernels employing busy-wait synchronization, DDOS identifies all busy-wait loops incurring no false detections. BOWS improves performance by 1.5× and reduces energy consumption by 1.6× versus Criticality-Aware Warp Acceleration (CAWA) [14].,,,,