What Scientific Applications can Benefit from Hardware Transactional Memory?

Achieving efficient and correct synchronization of multiple threads is a difficult and error-prone task at small scale and, as we march towards extreme scale computing, will be even more challenging when the resulting application is supposed to utilize millions of cores efficiently. Transactional Memory (TM) is a promising technique to ease the burden on the programmer, but only recently has become available on commercial hardware in the new Blue Gene/Q system and hence the real benefit for realistic applications has not been studied, yet. This paper presents the first performance results of TM embedded into OpenMP on a prototype system of BG/Q and characterizes code properties that will likely lead to benefits when augmented with TM primitives. We first, study the influence of thread count, environment variables and memory layout on TM performance and identify code properties that will yield performance gains with TM. Second, we evaluate the combination of OpenMP with multiple synchronization primitives on top of MPI to determine suitable task to thread ratios per node. Finally, we condense our findings into a set of best practices. These are applied to a Monte Carlo Benchmark and a Smoothed Particle Hydrodynamics method. In both cases an optimized TM version, executed with 64 threads on one node, outperforms a simple TM implementation. MCB with optimized TM yields a speedup of 27.45 over baseline