A Portability Layer of an All-pairs Operation for Hierarchical N-Body Algorithm Framework Tapas

Tapas is a C++ programming framework for developing hierarchical N-body algorithms such as Barnes-Hut and Fast Multipole Method, designed to experiment new implementations including even variations of tree traversals. A pairwise interaction calculation in N-body simulations, or an all-pairs operation, is an important part of Tapas for performance, which enables accelerations with GPUs. However, there is no commonly agreed all-pairs interface appropriate as a primitive, and moreover, it is not supported in existing data-parallel libraries for GPUs such as NVIDIA's Thrust. Thus, we designed an interface for an all-pairs operation that can be easily adopted in libraries and applications. Tapas's all-pairs has an extra function argument for flexibility, which corresponds to a consumer function of the result of an all-pairs that is missing in existing designs. This addition is not an ad hoc one, but it is guided by the consideration of algorithmic skeletons, which indicates the effect of the added argument cannot be substituted by the other arguments in general. The change is just adding an argument, but it gives flexibility to process the result, and the resulting implementation can attain almost the same performance as the tuned N-body implementation in the CUDA examples.