  We present Cholla (Computational Hydrodynamics On ParaLLel Architectures), a
new three-dimensional hydrodynamics code that harnesses the power of graphics
processing units (GPUs) to accelerate astrophysical simulations. Cholla models
the Euler equations on a static mesh using state-of-the-art techniques,
including the unsplit Corner Transport Upwind (CTU) algorithm, a variety of
exact and approximate Riemann solvers, and multiple spatial reconstruction
techniques including the piecewise parabolic method (PPM). Using GPUs, Cholla
evolves the fluid properties of thousands of cells simultaneously and can
update over ten million cells per GPU-second while using an exact Riemann
solver and PPM reconstruction. Owing to the massively-parallel architecture of
GPUs and the design of the Cholla code, astrophysical simulations with
physically interesting grid resolutions (> 256^3) can easily be computed on a
single device. We use the Message Passing Interface library to extend
calculations onto multiple devices and demonstrate nearly ideal scaling beyond
64 GPUs. A suite of test problems highlights the physical accuracy of our
modeling and provides a useful comparison to other codes. We then use Cholla to
simulate the interaction of a shock wave with a gas cloud in the interstellar
medium, showing that the evolution of the cloud is highly dependent on its
density structure. We reconcile the computed mixing time of a turbulent cloud
with a realistic density distribution destroyed by a strong shock with the
existing analytic theory for spherical cloud destruction by describing the
system in terms of its median gas density.
