Consolidated conflict detection for hardware transactional memory

Hardware Transactional Memory (HTM) promises to ease multithreaded parallel programming with uncompromised performance. Microprocessors supporting HTM implement a conflict detection mechanism to detect data access conflicts between transactions. Understanding the on-chip network bandwidth utilization of such mechanisms is important as the energy and latency cost of routing packets across the chip is growing alarmingly. We investigate the communication characteristics of a typical conflict detection mechanism. A variety of traffic overheads are identified, which accounts for a combined 56% of the total transactional traffic in a wide spectrum of applications. To combat this problem, we propose C2D (Consolidated Conflict Detection), a novel micro-architectural technique to consolidate conflict detection to a logically central (but physically distributed) agent to reduce the bandwidth utilization of conflict detection. Full system evaluation shows that the proposed technique, if applied to conventional eager conflict detection, can reduce 35% of the traffic and hence 27% of the network energy. The consolidated eager conflict detection generates less traffic than a lazy conflict detection scheme thereby closing the gap between bandwidth utilization of eager and lazy conflict detection.