IMPLEMENTING AN EFFICIENT COLLISIONLESS N-BODY CODE ON THE GRAY T3D

We discuss a parallel implementation of a collisionless N-body code on the Gray T3D. The code was previously successfully implemented on the Connection Machine CM-5. The algorithm evolves self-gravitating syste ms with approximate spheroidal symmetry by solving Poisson's equation for the global potential, given a realization of the density, and then advances the particle distribution under its own potential, solving f or the new potential iteratively, The algorithm is naturally parallel and can be efficiently implemented on current massively parallel archi tectures. We discuss some of the issues that arise in porting efficien t codes between different parallel architectures and show how changing the structure of the code can produce dramatic speedups on different architectures. We discuss some implications for automatic parallelizat ion and optimization with portable compilers. (C) 1997 American Instit ute of Physics.