DeTraS: Delaying Stores for Friendly-Fire Mitigation in Hardware Transactional Memory

Abstract

Commercial Hardware Transactional Memory (HTM) systems are best-effort designs that leverage the coherence substrate to detect conflicts eagerly. Resolving conflicts in favor of the requesting core is the simplest option for ensuring deadlock freedom, yet it is prone to livelocks. In this work, we propose and evaluate DeTraS (Delayed Transactional Stores), an HTM-aware store buffer design aimed at mitigating such livelocks. DeTraS takes advantage of the fact that modern commercial processors implement a large store buffer, and uses it to prevent transactional stores predicted to conflict from performing early in the transaction. By leveraging existing processor structures, we propose a simple design that improves the ability of requester-wins HTM systems to achieve forward progress in spite of high contention while side-stepping the performance penalty of falling back to mutual exclusion. With just over 50 extra bytes, DeTraS captures the advantages of lazy conflict management without the complexity brought into the coherence fabric by commit arbitration schemes nor the relaxation of the single-writer invariant of prior works. Through detailed simulations of a 16-core tiled CMP using gem5, we demonstrate that DeTraS brings reductions in average execution time of 25 percent when compared to an Intel RTM-like design.