Abstract
Abstract. A study of the scalability of the Finite-volumE Sea ice–Ocean circulation Model, Version 2.0 (FESOM2), the first mature global model of its kind formulated on unstructured meshes, is presented. This study includes an analysis of the main computational kernels with a special focus on bottlenecks in parallel scalability. Several model enhancements improving this scalability for large numbers of processes are described and tested. Model grids at different resolutions are used on four high-performance computing (HPC) systems with differing computational and communication hardware to demonstrate the model's scalability and throughput. Furthermore, strategies for improvements in parallel performance are presented and assessed. We show that, in terms of throughput, FESOM2 is on a par with state-of-the-art structured ocean models and, in a realistic eddy-resolving configuration (1/10∘ resolution), can achieve about 16 years per day on 14 000 cores. This suggests that unstructured-mesh models are becoming very competitive tools in high-resolution climate modeling. We show that the main bottlenecks of FESOM2 parallel scalability are the two-dimensional components of the model, namely the computations of the external (barotropic) mode and the sea-ice model. It is argued that these bottlenecks are shared with other general ocean circulation models.
Highlights
Mesoscale eddies play a critical role in the general circulation of the ocean, strongly affect biogeochemical processes, and impact marine life
In terms of throughput, FESOM2 is on a par with state-of-the-art structured ocean models and, in a realistic eddy-resolving configuration (1/10◦ resolution), can achieve about 16 years per day on 14 000 cores
We systematically explored the scalability of FESOM2 for a set of meshes (0.13 million, 0.64 million, and 5.6 million surface vertices, 47 layers) on several high-performance computing (HPC) systems and found a nearly linear scalability of FESOM2 code to the limit of 300–400 surface mesh vertices per core
Summary
Mesoscale eddies play a critical role in the general circulation of the ocean, strongly affect biogeochemical processes, and impact marine life. The main components of ocean circulation models limiting their scalability have been identified in the literature as the solver for the external (barotropic) mode (e.g., Huang et al, 2016; Prims et al, 2018) and the sea-ice model (e.g., Prims et al, 2018) They represent two-dimensional (2-D) stiff parts of the solution algorithm and require either linear solvers (usually iterative) or explicit pseudo-time-stepping with very small time steps (see the split-explicit method for the barotropic dynamic in Shchepetkin and McWilliams (2005) or elastic–viscous–plastic method for the sea ice in Hunke and Dukowicz (1997)). A discussion of results and strategies for future improvements of the parallel scaling of FESOM2 is presented in Sect. 6, followed by a brief conclusions section
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