Improving Utilization of Hardware Signatures in Transactional Memory

The transactional memory (TM) paradigm promises to increase programmer productivity by making it easier to write correct parallel programs. In fulfilling this goal, a TM system should maximize its performance with limited hardware resources. Conflict detection is an essential element for maintaining correctness among concurrent transactions in a TM system. Hardware signatures have been proposed as an area-efficient method for detecting conflicts. However, signatures can degrade TM performance by falsely declaring conflicts. Hence, improving the accuracy of signatures within a given hardware budget is a crucial issue for TM to be adopted as a mainstream programming model. In this paper, we propose a simple and effective signature design, the unified signature. Instead of using separate read- and write-signatures, we implement a single signature to track all read- and write-accesses. By merging read- and write-signatures, a unified signature can effectively enlarge the signature coverage without additional overhead. Within the constraints of a given hardware budget, a TM system with a unified signature outperforms a baseline system with the same-sized traditional signatures by reducing the number of falsely detected conflicts. Even though the unified signature scheme incurs read-read dependencies, we show that these false dependencies do not negate the benefit of unified signatures and can effectively be filtered out. A TM system with a 2-Kbit unified signature with a helper signature scheme achieves speedups of 15 percent over baseline TM with 33 percent less area and 49 percent less power.