A Parallel P^3M Code for Very Large Scale Cosmological Simulations
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abstract

We have developed a parallel Particle-Particle, Particle-Mesh (P^3M)
simulation code for the T3E well suited to studying the time evolution of
systems of particles interacting via gravity and gas forces in cosmological
contexts. The parallel code is based upon the public-domain serial Adaptive
P^3M code of Couchman et al(1). The algorithm resolves gravitational forces
into a long range component computed by discretizing the mass distribution and
solving Poisson's equation on a grid using an FFT convolution method, and a
short range component computed by direct force summation for sufficiently close
particle pairs. The code consists primarily of a particle-particle computation
parallelized by domain decomposition over blocks of neighbor-cells, a more
regular mesh calculation distributed in planes along one dimension, and several
transformations between the two distributions. Great care was taken throughout
to make optimal use of the available memory, so that the current implementation
is capable of simulating systems approaching 10^9 particles using a 1024^3 mesh
for the long range force computation. These are thus among the largest N-body
simulations ever carried out. We discuss these memory optimizations as well as
those motivated by computational performance. Results from production runs have
been very encouraging, and even prior to the implimentation of the full
adaptive scheme the code has been used effectively for simulations in which the
particle distribution becomes highly clustered as well as for other non-uniform
systems of astrophysical interest.