WarpX: implementation and performance on GPUs

WarpX is an advanced electromagnetic Particle-In-Cell code, and is part of the DoE Exascale Computing Project (ECP). The code provides many powerful features for large-scale simulations of plasmas (e.g. mesh refinement, load balancing, perfectly-matched layers), and in particular for intense laser-plasma interactions (e.g. boosted-frame, spectral solvers, quasi-cylindrical geometry). The code was recently ported to GPUs, and runs at scale on the Summit super- computer. We will describe the key components of the GPU implementation of WarpX, and how they allowed us to rapidly port the code while avoiding code duplication. We will also discuss the performance of the code on Summit, as well as the main limiting factors to overcome in order to reach additional speedup.