CERCS Technical Reports

GIT-CERCS-07-04

Adaptive Transaction Scheduling for Transactional Memory Systems

Transactional memory systems are expected to enable parallel programming at lower programming complexity, while delivering improved performance over traditional lock-based systems. Nonetheless, we observed that there are situations where transactional memory systems could actually perform worse, and that these situations will actually become dominant in future workloads as more and larger-scale transactional memory systems are available. Transactional memory systems can excel locks only when the executing workloads contain sufficient parallelism. When the workload lacks the inherent parallelism, blindly launching excessive transactions can adversely result in performance degradation. To quantitatively demonstrate the issues, we introduce the concept of effective transactions in this paper. We show that the effectiveness of a transaction is closely related to a dynamic quantity we call contention intensity. By limiting the contention intensity below the desired level, we can significantly increase transaction effectiveness. Increased effectiveness directly increases the overall performance of a transactional memory system. Based on our study, we implemented a transaction scheduler which not only guarantees that hardware transactional memory systems perform better than locks, but also significantly improves performance for both the hardware and software transactional memory systems.