Hybrid MPI-communication for the multi-angular SN parallel sweep on 3-D regular grids

The Koch-Baker-Alcouffe (KBA) algorithm has been widely used in solving the Boltzmann transport equation in parallel. To improve the scaling efficiency, the sweep is executed from the four corners of a 2-D processor map. Collisions take place when a processor has multiple tasks ready to be solved at one step. In order to handle these collisions, the 2-D processor map is divided into several zones that are defined as sub-processor maps with different octant sweep orders. In order to ensure the communications of these collided processors safe and efficient, four kinds of hybrid MPI-communication algorithms have been analyzed. Hybrid MPI-communications with processor slice achieve better scaling than the ones with block slice. The buffered-blocking communication with processor slice (BBP) achieves the best scaling efficiency. The comparisons of weak-scaling parallel efficiency between the standard KBA algorithm and BBP algorithm for multi-angular SN sweep have been made. The tests are performed on Tianhe-2 super computer by using 102-104 processors. For the test using 104 processors, the weak scaling efficiency is improved by over 13 percent compared to the standard KBA algorithm. The BBP and KBA algorithms have been applied to perform the pin-by-pin calculation of 3-D PWR core, which indicates that the BBP algorithm has the same accuracy with the standard KBA algorithm but better efficiency.