Directory-based conflict detection in hardware transactional memory

Abstract. One of the key design points of any hardware transactional memory (HTM) system is the conflict detection mechanism, and its effi-cient implementation becomes critical when conflicts are not a rare event. While many contemporary proposals rely on the coherence protocol to carry out conflict detection at the private cache levels, this approach is not optimal for systems that use a directory to maintain coherence over an unordered, scalable network, such as tiled CMPs. In this paper, we present a new scheme of conflict detection for HTM systems, which moves this key mechanism from the private caches to the directory level. We propose a novel transactional book-keeping method and describe how this detection can be carried out more efficiently at the directory. Simula-tion results show that our approach obtains reductions in execution time between 25 and 55 % for transactional benchmarks with a high number of conflicts, with an average improvement over LogTM-SE of 15%.