Model Optimisation
The cost of the Stratosphere-Troposphere chemistry scheme in UKCA using the Newton-Raphson solver with the on-line photolysis scheme Fast-jX relative to the climate model HadGEM3-A is as follows:
| Model | PEs | OpenMP Threads | Time Elapsed (sec) |
|---|---|---|---|
| HadGEM3-A | 8x16 | 1 | 3798 |
| HadGEM3-A + StratTrop(N-R) + Fast-jX | 8x16 | 1 | 15602 |
| HadGEM3-A | 8x16 | 2 | 2328 |
| HadGEM3-A + StratTrop(N-R) + Fast-jX | 8x16 | 2 | 11730 |
On a 8x16 PE configuration and 1 OpenMP thread on the Monsoon facility or the Met Office's Power6 IBM (hpc1e/1f), UKCA and Fast-jX together add 310% to the cost of HadGEM3-A. However, with 2 OpenMP threads and no OpenMP compiler directives in UKCA, the relative cost of UKCA is even higher, adding approximately 400% to the cost of HadGEM3-A. Adding aerosol chemistry and UKCA-MODE aerosols will make it even more costly. Therefore, there is a clear need for optimisation.
As part of the HadGEM3-ES development project, currently being led by F. O'Connor, there are plans to do some optimisation work on UKCA. In particular, a more complete assessment of the model cost will be carried out and the potential speedup which may be gained from simple code re-writing, load balancing, and the use of OpenMP, dedicated I/O servers and maths libraries will be explored. Some scientific optimisation such as throwing out unwanted reactions (or species), tweaking chemistry to improve convergence, etc.
should also be considered. The use of an alternative solver, such as a Rosenbrock solver, may also be investigated.
