CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

COUPLING TOOLS AND HPC CLIMATE MODELING
Another asset of this simplified configuration is its lack of climate biases of large amplitude due to the flux
corection implementation. The energetic balance of the model, both at the top of atmosphere and air-sea
interface, is quickly reached without the need of exhaustive parameter sensitivity experiments. This allows
a quick and robust estimation of the similarity of simulated climates on different architectures which is a
necessary condition in a machine to machine porting context.
In the framework of the FP7 European project IS-ENES, the ARPEGE-NEMIX model has been successfully
implemented and tested on a broad selection of scalar supercomputers, from which a PRACE tier-0 machine
([CC8]). Efficient weak scaling of both ARPEGE and NEMIX models allows the use of more than 1000
cores. The performed benchmarks suggest that decadal-scale climate experiments at such resolution can
now be realistically processed on such machines (less than 5 days necessary to simulate 10 years of climate).
4.3 Coupling technical aspects and fault tolerance compliance
Preliminary simulations showed the lack of performance of the OASIS3 coupling implementation :
measured on a set of European coupled GCMs, from 1 to 20 percent of the total duration could be
spend on coupling exchanges ([CC16], [CC24]). First, an OASIS4 alternative has been explored, using
our model and several other CGCMs, revealing other incompatibilities ([CC25]). At the moment, a newly
developed OASIS3-MCT solution is tested, to try to enhance coupler scalability. Furthermore and based
on the practical IS-ENES funded exercice of coupler interface coding and performances analysis ([CC26]),
a testbed for a comparison of the NCAR(flux coupler) and CERFACS(OASIS) coupling environment has
been suggested, revealing the strong and necessary interaction between OASIS development and beta testing
during OASIS model interface implementation on HPC configurations.
The performed intermediate high resolution experiments revealed new bottlenecks such as :
– Output speed decrease
– Data storage capabilities
– Runtime environnement unstability
which can strongly impact the overall performance of our climate numerical laboratory. New data
production paradigms are currently tested by CERFACS partners (IPSL) and started to be included within
our own HPC configurations. In parallel, we investigated the reduction of the amount of produced climate
data using traditional (interpolation) and new (reduction) schemes.
Concerning runtime environment stability and to start exploring fault tolerance compliance of existing
climate models, a study with the NEMO code has started on how to adapt an error handling that could
reorganize MPI communications after failure and keep calulating on the remaining process. This work is
part of code adaptation for HPC, developed in the framework of ANR SPADES jointly with INRIA Saclay
laboratory (LRI).
CERFACS ACTIVITY REPORT 109