A Comparison of Performance Tunabilities between OpenCL and OpenACC

To design and develop any auto tuning mechanisms for OpenACC, it is important to clarify the differences between conventional GPU programming models and OpenACC in terms of available programming and tuning techniques, called performance tunabilities. This paper hence discusses the performance tunabilities of OpenACC and OpenCL. As OpenACC cannot synchronize threads running on GPUs, some important techniques are not available to OpenACC. Therefore, we also design an additional compiler directive for thread synchronization. Evaluation results show that both OpenCL and OpenACC need architecture-aware optimizations, and similar approaches to performance optimization are effective for both OpenCL and OpenACC. The additional directive can allow OpenACC to describe more tuning techniques in the same approach as OpenCL. As it is obvious that OpenACC is more productive than OpenCL especially for legacy application migration, OpenACC is a very promising programming model if it can achieve the same performance as the conventional GPU programming models such as CUDA and OpenCL.