2015MNRAS.450.4070W

Accurate direct N-body simulations help to obtain detailed information about the dynamical evolution of star clusters. They also enable comparisons with analytical models and Fokker-Planck or Monte Carlo methods. nbody6 is a well-known direct N-body code for star clusters, and nbody6++ is the extended version designed for large particle number simulations by supercomputers. We present nbody6++gpu, an optimized version of nbody6++ with hybrid parallelization methods (MPI, GPU, OpenMP, and AVX/SSE) to accelerate large direct N-body simulations, and in particular to solve the million-body problem. We discuss the new features of the nbody6++gpu code, benchmarks, as well as the first results from a simulation of a realistic globular cluster initially containing a million particles. For million-body simulations, nbody6++gpu is 400-2000 times faster than nbody6 with 320 CPU cores and 32 NVIDIA K20X GPUs. With this computing cluster specification, the simulations of million-body globular clusters including 5 percent primordial binaries require about an hour per half-mass crossing time.