On Approximate Speculative Lock Elision

Abstract

Each synchronization point represents a point of serialization, and thereby can easily hurt parallel scalability. As demonstrated by recent studies, approximating, i.e., relaxing synchronization by eliminating a subset of synchronization points spatio-temporally can help improve parallel scalability, as long as approximation incurred violations of basic execution semantics remain predictable and controllable. Even if the divergence from fully-synchronized execution renders lower computation accuracy ratherthan catastrophic program termination, for approximation to be viable, the accuracy loss must be bounded. In this paper, we assess the viability of approximate synchronization using Speculative Lock Elision (SLE), which was adopted by hardware transactional memory implementations from industry, as a baseline for comparison. Specifically, we investigate the efficacy of exploiting semantic and temporal characteristics of critical sections in preventing excessive loss in computation accuracy, and devise a light-weight, proof-of-concept Approximate Speculative Lock Elision (ASLE) implementation, which exploits existing hardware support for SLE.