A Review of The Challenges and Results of Refactoring the Community Climate Code COSMO for Hybrid Cray HPC Systems

We summarize the results of porting the numerical weather simulation code COSMO to different hybrid Cray HPC systems. COSMO was written in Fortran with MPI, and the aim of the refactoring was to support both many-core systems and GPU-accelerated systems with minimal disruption to the user community. With this in mind, different approaches were taken to refactor the different components of the code: the dynamical core was refactored with a C++-based domain specific language for structured grids which provides both CUDA and OpenMP back ends; and the physical parameterizations were refactored by adding OpenACC and OpenMP directives to the original Fortran code. This report gives a detailed description of the challenges presented by such a large refactoring effort using different languages on Cray systems, along with performance results on three different Cray systems at CSCS: Rosa (XE6), Todi (XK7) and Daint (XC30).