Critical Sections and Software Transactional Memory Comparison in the Context of a TLS Runtime Library

Abstract

Transactional Memory (TM) is a technique that aims to mitigate the performance losses that are inherent to the serialization of accesses in critical sections. Some studies have shown that the use of TM may lead to performance improvements, despite the existence of management overheads. However, the relative performance of TM, with respect to classical critical sections management depends greatly on the actual percentage of times that the same data is handled simultaneously by two transactions. In this paper, we compare the relative performance of the critical sections provided by OpenMP with respect to two Software Transactional Memory (STM) implementations. These three methods are used to manage concurrent data accesses in ATLaS, a software-based, Thread-Level Speculation (TLS) system. The complexity of this application makes it extremely di cult to predict whether two transactions may conflict or not, and how many times the transactions will be executed. Our experimental results show that the STM solutions only deliver a performance comparable to OpenMP when there are almost no conflicts. In any other case, their performance losses make OpenMP the best alternative to manage critical sections.