Revisiting phased transactional memory

Abstract

In recent years, Hybrid TM (HyTM) has been proposed as a transactional memory approach that leverages on the advantages of both hardware (HTM) and software (STM) execution modes. HyTM assumes that concurrent transactions can have very different phases and thus should run under different execution modes. Although HyTM has shown to improve performance, the overall solution can be complicated to manage, both in terms of correctness and performance. On the other hand, Phased Transactional Memory (PhTM) considers that concurrent transactions have similar phases, and thus all transactions could run under the same mode. As a result, PhTM does not require coordination between transactions on distinct modes making its implementation simpler and more exible. In this paper we claim that PhTM is a competitive alternative to HyTM and propose PhTM∗, the first implementation of PhTM on modern HTM-ready processors. PhTM∗novelty relies in avoiding unnecessary transitions to software mode by: (i) taking into account the categories of hardware aborts; (ii) adding a new serialization mode. Experimental results with Haswell's TSX reveal that, for the STAMP benchmark suite, PhTM∗performs on average 11% better than PhTM, a previous phase-based TM, and 15% better than HyTM-NOrec, a state-of-the-art HyTM. In addition, PhTM∗showed to be even more effective running on a Power8 machine by performing over 25% and 36% better than PhTM and HyTM-NOrec, respectively.