  We present direct astrophysical N-body simulations with up to a few million
bodies using our parallel MPI/CUDA code on large GPU clusters in China, Ukraine
and Germany, with different kinds of GPU hardware. These clusters are directly
linked under the Chinese Academy of Sciences special GPU cluster program in the
cooperation of ICCS (International Center for Computational Science). We reach
about the half the peak Kepler K20 GPU performance for our phi-GPU code [2], in
a real application scenario with individual hierarchically block time-steps
with the high (4th, 6th and 8th) order Hermite integration schemes and a real
core-halo density structure of the modeled stellar systems. The code and
hardware are mainly used to simulate star clusters [23, 24] and galactic nuclei
with supermassive black holes [20], in which correlations between distant
particles cannot be neglected.
