Performance Analysis of EC-EARTH 3.1

The EC-EARTH model is a global, coupled climate model that consists of the separate components IFS for the
atmosphere and NEMO for the ocean that are coupled using the OASIS coupler. EC-EARTH was ported and run on
the Curie system. Different configurations, using resolutions from T159 (approx. 128km) to T799 (approx 25km),
were available for benchmarking. Scalasca was used to analyze the performance of the model in detail. Although it
was expected that either the I/O or the coupling would be a bottleneck for scaling of the highest resolution model,
that is clearly not, yet, the case. The IFS model uses two MPI_Alltoallv calls per timestep that dominate the loss of
scaling at 1024 cores. Using the OpenMP functionality in IFS could potentially increase scalability considerably, but
this does not yet work on Curie. Work is ongoing to make MPI_Alltoallv more efficient on Curie. It is expected that
I/O and/or coupling does become a bottleneck when IFS can be scaled further than 2000 cores. Therefore, the
OASIS team increased the scalability of OASIS dramatically with the implementation of a radically different
approach, showing less than 1% overhead at 2000 cores. The scalability of NEMO was improved during an earlier
PRACE project. The I/O subsystem in IFS is described and is probably not easily accelerated unless it is being
rewritten and uses a different file format.