# Targeting GPUs with OpenMP Directives on Summit: A Simple and Effective Fortran Experience

## TL;DR

This paper demonstrates a simplified approach using OpenMP directives to effectively utilize GPUs on Summit, achieving significant speedups and scalable performance for astrophysics and fluid dynamics applications.

## Contribution

It introduces a streamlined method for GPU offloading with OpenMP in Fortran, enhancing ease of use and performance on Summit supercomputer.

## Key findings

- Achieved ~12x speedup with GPUs on Summit.
- Demonstrated reasonable weak scaling up to 8000 GPUs.
- Provided open-source code for GPU targeting with OpenMP.

## Abstract

We use OpenMP to target hardware accelerators (GPUs) on Summit, a newly deployed supercomputer at the Oak Ridge Leadership Computing Facility (OLCF), demonstrating simplified access to GPU devices for users of our astrophysics code GenASiS and useful speedup on a sample fluid dynamics problem. We modify our workhorse class for data storage to include members and methods that significantly streamline the persistent allocation of and association to GPU memory. Users offload computational kernels with OpenMP target directives that are rather similar to constructs already familiar from multi-core parallelization. In this initial example we ask, "With a given number of Summit nodes, how fast can we compute with and without GPUs?", and find total wall time speedups of $\sim 12\mathrm{X}$. We also find reasonable weak scaling up to 8000 GPUs (1334 Summit nodes). We make available the source code from this work at https://github.com/GenASiS/GenASiS_Basics.