Optimization of a Legacy Open Source Cfd Code for the New High Performance Computing Architectures

Legacy computational fluid dynamics (CFD) software plays a crucial role in today’s simulation-based engineering due to the confidence level established through validation of these codes over time. Fortran, one of the oldest programming languages around, has gained an important role in high performance computing (HPC) environment as the complexity of the problems tackled increased with their size. As HPC has rapidly changed, Fortran has also evolved and modern Fortran (e.g., Fortran 2003, and 2008 standards) has shown that it can remain the choice for engineering and scientific codes among other modern programming languages like C++. However, modernizing legacy CFD software based on older Fortran versions has been known to pose its own unique challenges. For example, refactoring the code by eliminating obsolete constructs and revising the data structures to employ modern Fortran features may require significant rewrite of the entire code. Such rewrite requires standalone verification and validation phases to establish the credibility of the revised code. On the other hand, there is a strong motivation and substantial need to optimize the performance of legacy CFD codes on the new high performance computing platforms in order to take advantage of the potential performance gains. The rapidly evolving and increasingly heterogeneous processor architectures with the new memory technologies and communication over high speed but networks, pose a diverse range of challenges for legacy CFD code developers to optimize the performance of their software for the targeted HPC architectures. In addition, features such as extensive paralellism through many cores (threads) as well as vector capabilities of modern HPC platforms require significant changes to legacy codes to enable parallelism and vectorization that can result in orders of magnitude higher performance. In this study, we present the preliminary results of our optimization efforts on a partially modernized legacy CFD code, MFIX for the new Intel R © Many Integrated Core (MIC) architecture based HPC systems. This effort, which started during the first Hack-athon organized by the National Energy Research Scientific Computing Center (NERSC)