Performance Portability Challenges for Fortran Applications

This project investigates how different approaches to parallel optimization impact the performance portability for Fortran codes. In addition, we explore the productivity challenges due to the software tool-chain limitations unique to Fortran. For this study, we build upon the Truchas software, a metal casting manufacturing simulation code based on unstructured mesh methods and our initial efforts for accelerating two key routines, the gradient and mimetic finite difference calculations. The acceleration methods include OpenMP, for CPU multi-threading and GPU offloading, and CUDA for GPU offloading. Through this study, we find that the best optimization approach is dependent on the priorities of performance versus effort and the architectures that are targeted. CUDA is the most attractive where performance is the main priority, whereas the OpenMP on CPU and GPU approaches are preferable when emphasizing productivity. Furthermore, OpenMP for the CPU is the most portable across architectures. OpenMP for CPU multi-threading yields 3%-5% of achievable performance, whereas the GPU offloading generally results in roughly 74%-90% of achievable performance. However, GPU offloading with OpenMP 4.5 results in roughly 5% peak performance for the mimetic finite difference algorithm, suggesting further serial code optimization to tune this kernel. In general, these results imply low performance portability, below 10% as estimated by the Pennycook metric. Though these specific results are particular to this application, we argue that this is typical of many current scientific HPC applications and highlights the hurdles we will need to overcome on the path to exascale.