The Case for Phase-Based Transactional Memory

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 flexible. In this article we make the case for phase-based transactional systems using PhTM*, the first implementation of PhTM on modern HTM-ready processors. PhTM* novelty relies on avoiding unnecessary transitions to software mode by: (i) taking into account the categories of hardware aborts; (ii) adding a new serialization mode. Experimental results using Broadwell's TSX reveal that, for the STAMP benchmark suite, PhTM* performs on average 1.68x better than PhTM, a previous phase-based TM, 2.08x better than HyTM-NOrec, a state-of-the-art HyTM, and 2.28x better than HyCO, the most recent hybrid system in the literature. In addition, PhTM* also showed to be effective when running on a Power8 machine by performing over 1.18x, 1.36x and 1.81x better than PhTM, HyTM-NOrec and HyCO, respectively. We also show that STAMP applications do not exhibit hybrid behavior to justify the use of conventional hybrid systems, thus making PhTM* a better solution to those type of programs. Finally, we show for the first time that conventional hybrid systems do not perform better than phased-based system in a scenario with hybrid-behaved transactions.