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Wednesday, July 29 • 10:30am - 11:00am
Autotuning OpenACC Work Distribution via Direct Search

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OpenACC provides a high-productivity API for programming GPUs and similar accelerator devices. One of the last steps in tuning OpenACC programs is selecting values for the num_gangs and vector_length clauses, which control how a parallel workload is distributed to an accelerator's processing units. In this paper, we present an autotuner that can assist the programmer in selecting high-quality values for these parameters, and we evaluate the effectiveness of two direct search methods in finding solutions. We compared num_gangs and vector_length values found by our autotuner to those found by a bounded exhaustive search. On a suite of 36 OpenACC kernels, one or both of our autotuner's direct search methods identified values within the top 5% for 29 of the kernels; all results were within the top 30%. The majority of the kernels achieved a statistically significant speedup over the compiler's defaults, and the autotuner required only 7-11 runs of the target program, on average.

Wednesday July 29, 2015 10:30am - 11:00am CDT
Majestic B

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