Mitigating the Mismatch between the Coherence Protocol and Conflict Detection in Hardware Transactional Memory

Abstract

Hardware Transactional Memory (HTM) usually piggybacks onto the cache coherence protocol to detect data access conflicts between transactions. We identify an intrinsic mismatch between the typical coherence scheme and transaction execution, which causes a sizable amount of unnecessary transaction aborts. This pathological behavior is called false aborting and increases the amount of wasted computation and on-chip communication. For the TM applications we studied, 41% of the transactional write requests incur false aborting. To combat false aborting, we propose Predictive Unicast and Notification (PUNO), a novel hardware mechanism to 1) replace the inefficient coherence multicast with a unicast scheme to prevent transactions from being disrupted unnecessarily and 2) restrain transaction polling through proactive notification. PUNO reduces transaction aborts by 61% and network traffic by 32% in workloads representative of future TM applications with a VLSI implementation area overhead of 0.41%.