Load balance and parallel I/O: Optimising COSA for large simulations

This paper presents the optimisation of the parallel functionalities of the Navier-Stokes Computational Fluid Dynamics research code COSA, a finite volume structured multi-block code featuring a steady solver, a general purpose time-domain solver, and a frequency-domain harmonic balance solver for the rapid solution of unsteady periodic flows. The optimisation focuses on improving the scalability of the parallel input/output functionalities of the code and developing an effective and user-friendly load balancing approach. Both features are paramount for using COSA efficiently for large-scale production simulations using tens of thousands of computational cores. The efficiency enhancements resulting from optimising the parallel I/O functionality and addressing load balance issues has provided up to a 4× performance improvement for unbalanced simulations, and 2× performance improvements for balanced simulations.