  We present performance measurements of direct gravitational N -body
simulation on the grid, with and without specialized (GRAPE-6) hardware. Our
inter-continental virtual organization consists of three sites, one in Tokyo,
one in Philadelphia and one in Amsterdam. We run simulations with up to 196608
particles for a variety of topologies. In many cases, high performance
simulations over the entire planet are dominated by network bandwidth rather
than latency. With this global grid of GRAPEs our calculation time remains
dominated by communication over the entire range of N, which was limited due to
the use of three sites. Increasing the number of particles will result in a
more efficient execution. Based on these timings we construct and calibrate a
model to predict the performance of our simulation on any grid infrastructure
with or without GRAPE. We apply this model to predict the simulation
performance on the Netherlands DAS-3 wide area computer. Equipping the DAS-3
with GRAPE-6Af hardware would achieve break-even between calculation and
communication at a few million particles, resulting in a compute time of just
over ten hours for 1 N -body time unit. Key words: high-performance computing,
grid, N-body simulation, performance modelling
