Don't Forget About Synchronization!

Heterogeneous devices are becoming necessary components of high performance computing infrastructures, and the graphics processing unit (GPU) plays an important role in this landscape. Given a problem, the established approach for exploiting the GPU is to design solutions that are parallel, without data or flow dependencies. These solutions are then offloaded to the GPU's massively parallel capability. This design principle (i.e., avoiding contention) often leads to developing applications that cannot maximize GPU hardware utilization. The goal of this paper is to challenge this common belief by empirically showing that allowing even simple forms of synchronization enables programmers to design parallel solutions that admit conflicts and achieve better utilization of hardware parallelism. Our experience shows that lock-based solutions to the k-means clustering problem outperform the well-engineered and parallel KMCUDA on both synthetic and real datasets; averaging 8.4x faster runtimes at high contention and 8.1x faster for low contention, with maximums of 25.4x and 74x, respectively. We summarize our findings by identifying two guidelines to help make concurrency effective when programming GPU applications.