OpenACC and OpenMP-Accelerated Fortran/C++ Gyrokinetic Fusion Code GENE-X for Heterogeneous Architectures

Achieving net-positive fusion energy and its commercialization requires not only engineering marvels but also state-of-the-art, massively parallel codes that can handle reactor-scale simulations. The GENE-X code is a global continuum gyrokinetic turbulence code designed to predict energy confinement and heat exhaust for future fusion reactors. GENE-X is capable of simulating plasma turbulence from the core region to the wall of a magnetic confinement fusion (MCF) device. Originally written in Fortran 2008, GENE-X leverages MPI+OpenMP for parallel computing. In this paper, we augment the Fortran-based compute operators in GENE-X to a C++-17 layer exposing them to a wide array of C++-compatible tools. Here we focus on offloading the augmented operators to GPUs via directive-based programming models such as OpenACC and OpenMP offload. The performance of GENE-X is comprehensively characterized, e.g., by roofline analysis on a single GPU and scaling analysis on multi-GPUs. The major compute operators achieve significant performance improvements, shifting the bottleneck to inter-GPU communications. We discuss additional opportunities to further enhance the performance, such as reducing memory traffic and improving memory utilization efficiency.