Iterative Schedule Optimization for Parallelization in the Polyhedron Model
Stefan Ganser, Armin Größlinger, Norbert Siegmund, Sven Apel, Christian Lengauer · ACM Transactions on Architecture and Code Optimization · 2017
The polyhedron model is a powerful model to identify and apply systematically loop transformations that improve data locality (e.g., via tiling) and enable parallelization. In the polyhedron model, a loop transformation is, essentially, represented as an affine function. Well-established algorithms for the discovery of promising transformations are based on performance models. These algorithms have the drawback of not being easily adaptable to the characteristics of a specific program or target hardware. An iterative search for promising loop transformations is more easily adaptable and can help to learn better models. We present an iterative optimization method in the polyhedron model that targets tiling and parallelization. The method enables either a sampling of the search space of legal loop transformations at random or a more directed search via a genetic algorithm. For the latter, we propose a set of novel, tailored reproduction operators. We evaluate our approach against existing iterative and model-driven optimization strategies. We compare the convergence rate of our genetic algorithm to that of random exploration. Our approach of iterative optimization outperforms existing optimization techniques in that it finds loop transformations that yield significantly higher performance. If well configured, then random exploration turns out to be very effective and reduces the need for a genetic algorithm.