GraviDy, a GPU modular, parallel direct-summation $N-$body integrator:
  Dynamics with softening
  A wide variety of outstanding problems in astrophysics involve the motion of
a large number of particles ($N\gtrsim 10^{6}$) under the force of gravity.
These include the global evolution of globular clusters, tidal disruptions of
stars by a massive black hole, the formation of protoplanets and the detection
of sources of gravitational radiation. The direct-summation of $N$
gravitational forces is a complex problem with no analytical solution and can
only be tackled with approximations and numerical methods. To this end, the
Hermite scheme is a widely used integration method. With different numerical
techniques and special-purpose hardware, it can be used to speed up the
calculations. But these methods tend to be computationally slow and cumbersome
to work with. Here we present a new GPU, direct-summation $N-$body integrator
written from scratch and based on this scheme. This code has high modularity,
allowing users to readily introduce new physics, it exploits available
high-performance computing resources and will be maintained by public, regular
updates. The code can be used in parallel on multiple CPUs and GPUs, with a
considerable speed-up benefit. The single GPU version runs about 200 times
faster compared to the single CPU version. A test run using 4 GPUs in parallel
shows a speed up factor of about 3 as compared to the single GPU version. The
conception and design of this first release is aimed at users with access to
traditional parallel CPU clusters or computational nodes with one or a few GPU
cards.
