Optimum number of CPU-s to be used?

[jayasankar]

Is there a thumb rule or an expression for the optimum number of CPUs to be used for a ROMS simulation?

[kate]

It depends on so many variables. It's best just to do some tests on your own problem using different numbers of cores. It mostly depends on your grid - too many cores and communications dominate the run time, too few and you don't fit into cache.