Quokka: a code for two-moment AMR radiation hydrodynamics on GPUs

We present quokka, a new subcycling-in-time, block-structured adaptive mesh refinement (AMR) radiation hydrodynamics (RHD) code optimized for graphics processing units (GPUs). quokka solves the equations of HD with the piecewise parabolic method (PPM) in a method-of-lines formulation, and handles radiative transfer via the variable Eddington tensor (VET) radiation moment equations with a local closure. We use the amrex library to handle the AM management. In order to maximize GPU performance, we combine explicit-in-time evolution of the radiation moment equations with the reduced speed-of-light approximation. We show results for a wide range of test problems for HD, radiation, and coupled RHD. On uniform grids in 3D on a single GPU, our code achieves >250 million hydrodynamic updates per second and almost 40 million radiation hydrodynamic updates per second. For RHD problems on uniform grids in 3D, our code scales from 4 to 256 GPUs with an efficiency of 76 per cent. The code is publicly released under an open-source license on GitHub.