On The Algorithmic Aspects Of Using Openmp Synchronization Mechanisms: The Effects Of Transactional Memory

In this paper we analyze the effects of using different OpenMP synchronization mechanisms in iterative mesh optimization algorithms run on the IBM Blue Gene/Q system. We perform a systematic study of a threaded Laplacian mesh smoothing method on Cartesian meshes of different sizes that have been initially perturbed by a factor that is random, but within a controlled range. We consider three different run modes, two of which are OpenMP synchronization mechanisms: (hardware) transactional memory (TM), OpenMP critical, and "none". We find that TM typically outperforms the other two modes in terms of its convergence characteristics. Because of the algorithmic simplicity and light operation count, the raw runtime performance was not our focus in this work; however, we present some results on TM scaling. We also show the TM rollback and conflict probabilities, and conclude that mesh optimization codes are good candidates for using TM when the more general "time-to-convergence" criterion is considered.