code_saturne User's Forum

Hello,

I noticed somewhere in the forum that the parallelism efficiency with the legacy FV method is reached with about 32000 cells per CPU (assumed without multi-threading). Is that the same ratio using the CS default CDO_fb space discretization ?

Thank you in advance.

Kind regards.

Hello,

This value is really an average, an depends on the network and processor speeds/balance, and number of nodes used. On a single node, scalabilty may be better, while on billion-cell cases running on thousands of cores, the optimum is often higher.

So for legacy FV, I always recommend starting from 30 to 50 cells per core, and running a few iterations using both half and double that. Than repeat in the direction where scalability seems best, until things seem about optimal.

For CDO, scalability might be slightly better, though I recommend using the same approach. Using hybrid MPI/OpenMP might slightly improve things at high core counts.

Processor architecture is also important. Some years ago, I saw a case on a workstation loose scalability very fast on my desktop machine, with almost no improvement from 2 to 8 cores (for 200000 cells), while the same binary had a speedup of about 7 on 8 cores on a more powerful workstation (both Intel Xeon processors, but different models). So you need to test on your own case to be sure (but so get back to me if performance is completely different from that expected).

Best regards,

Yvan