24.77 Pflops on a Gravitational Tree-Code to Simulate the Milky Way
  Galaxy with 18600 GPUs
  We have simulated, for the first time, the long term evolution of the Milky
Way Galaxy using 51 billion particles on the Swiss Piz Daint supercomputer with
our $N$-body gravitational tree-code Bonsai. Herein, we describe the scientific
motivation and numerical algorithms. The Milky Way model was simulated for 6
billion years, during which the bar structure and spiral arms were fully
formed. This improves upon previous simulations by using 1000 times more
particles, and provides a wealth of new data that can be directly compared with
observations. We also report the scalability on both the Swiss Piz Daint and
the US ORNL Titan. On Piz Daint the parallel efficiency of Bonsai was above
95%. The highest performance was achieved with a 242 billion particle Milky Way
model using 18600 GPUs on Titan, thereby reaching a sustained GPU and
application performance of 33.49 Pflops and 24.77 Pflops respectively.
