  Lattice QCD calculations were one of the first applications to show the
potential of GPUs in the area of high performance computing. Our interest is to
find ways to effectively use GPUs for lattice calculations using the overlap
operator. The large memory footprint of these codes requires the use of
multiple GPUs in parallel. In this paper we show the methods we used to
implement this operator efficiently. We run our codes both on a GPU cluster and
a CPU cluster with similar interconnects. We find that to match performance the
CPU cluster requires 20-30 times more CPU cores than GPUs.
