PhTM: Phased Transactional Memory

Hybrid transactional memory (HyTM) [3] works in today’s systems, and can use future “best effort” hardware transactional memory (HTM) support to improve performance. Best effort HTM can be substantially simpler than alternative “unbounded” HTM designs being proposed in the literature, so HyTM both supports and encourages an incremental approach to adopting HTM. We introduce Phased Transactional Memory (PhTM), which supports switching between different “phases”, each implemented by a different form of transactional memory support. This allows us to adapt between a variety of different transactional memory implementations according to the current environment and workload. We describe a simple PhTM prototype, and present experimental results showing that PhTM can match the performance and scalability of unbounded HTM implementations better than our previous HyTM prototype when best effort HTM support is available and effective, and is more competitive with state-of-the-art software transactional memory implementations when it is not.