Cholla - MHD: An Exascale - Capable Magnetohydrodynamic Extension to the Cholla Astrophysical Simulation Code

We present an extension of the massively parallel, GPU native, astrophysical hydrodynamics code Cholla to magnetohydrodynamics (MHD). Cholla solves the ideal MHD equations in their Eulerian form on a static Cartesian mesh utilizing the Van Leer + Constrained Transport integrator, the HLLD Riemann solver, and reconstruction methods at second and third order. Cholla's MHD module can perform over 200 million cell updates per GPU-second while using the HLLD Riemann solver and second order reconstruction. The inherently parallel nature of GPUs combined with increased memory in new hardware allows Cholla's MHD module to perform simulation with resolutions of $>450^3$ cells on a single GPU. We employ GPU direct MPI to attain nearly perfect weak scaling on the exascale supercomputer \textit{Frontier}, while using up to 74,000 GPUs and simulating a total grid size of over 1.2 trillion cells. A suite of test problems highlights the accuracy of Cholla's MHD module and demonstrates that zero magnetic divergence in solutions is maintained to round off error. We also present new testing and continuous integration tools using GoogleTest, GitHub Actions, and Jenkins that have made development more robust and accurate and ensure reliability in the future.