Parallel algorithms for transport sweeps on unstructured meshes

The Boltzmann Transport Equation is solved on unstructured meshes using the Discrete Ordinates Method. The flux for each ordinate is swept across the computational grid, within a source iteration loop that accounts for the coupling between the different ordinates. In this paper, a spatial domain decomposition strategy is used to divide the work among the available CPUs. The sequential nature of the sweep process makes the parallelization of the overall algorithm the most challenging aspect. Several parallel sweep algorithms, which represent different options of interleaving communications and calculations in the solution process, are analysed. The option of grouping messages by means of buffering is also considered. One of the heuristics proposed consistently stands out as the best option in all the situations analyzed, which include different geometries and different sizes of the ordinate set. With this algorithm, good scalability results have been achieved regarding both weak and strong speedup tests with up to 2560 CPUs.