On the interactions between ILP and TLP with hardware transactional memory

Abstract

Hardware implementations of Transactional Memory (HTM) are designed to facilitate efficient thread synchronization in parallel programs, encouraging the use of larger critical sections. By employing optimistic concurrency control to execute transactions speculatively, HTM systems promise to deliver the performance benefits typically associated with fine-grained locks. In doing so, HTM systems must deal with transaction aborts. While under certain conditions aborts may be caused by the inherent limitations of hardware structures employed to implement TM (e.g., caches), conflicting concurrent accesses to shared memory locations are generally the prevailing cause for squashing the work done by a transactionIn this study, we present what we believe to be, to the best of our knowledge, the first characterization of how the aggressiveness of processor cores, particularly their ability to exploit instruction-level parallelism (ILP), interacts with the support for optimistic thread-level speculation offered by HTM systems. We have observed that by adjusting the size of structures that facilitate out-of-order and speculative execution, the number of aborts in the execution of transactional workloads can be altered in best-effort HTM implementations. Our findings indicate that in scenarios with high contention, a smaller number of powerful cores is more suitable, whereas in low contention scenarios, using a larger number of less aggressive cores is preferable. In addition, HTM systems that employ lazy detection and those employing eager detection with requester-stalls resolution, benefit from using simpler cores. In conclusion, abort ratios can be reduced with a careful choice of both processor aggressiveness and design aspects for each application depending on its contention.