A Dynamically Adaptable Hardware Transactional Memory

Most Hardware Transactional Memory (HTM) implementations choose fixed version and conflict management policies at design time. While eager HTM systems store transactional state in-place in memory and resolve conflicts when they are produced, lazy HTM systems buffer the transactional state in specialized hardware and defer the resolution of conflicts until commit time. Each scheme has its strengths and weaknesses, but, unfortunately, both approaches are too inflexible in the way they manage data versioning and transactional contention. Thus, fixed HTM systems may result in a significant performance opportunity loss when they execute complex transactional applications. In this paper, we present DynTM (Dynamically Adaptable HTM), the first fully-flexible HTM system that permits the simultaneous execution of transactions using complementary version and conflict management strategies. In the heart of DynTM is a novel coherence protocol that allows tracking conflicts among eager and lazy transactions. Both the eager and the lazy execution modes of DynTM exhibit very high performance compared to modern HTM systems. For example, the DynTM lazy execution mode implements local commits to improve on previous proposals. In addition, lazy transactions share the majority of hardware support with eager transactions, reducing substantially the hardware cost compared to other lazy HTM systems. By utilizing a simple predictor to decide the best execution mode for each transaction at runtime, DynTM obtains an average speedup of 34% over HTM systems that employ fixed version and conflict management policies.