How to set NTASKS value

[bidyut]

Hi, I am trying to set NTASKS=3060 for --compset FHIST --res f09_f09_mg17. But when I run ./case.build, I am getting error :
"err=CAM build-namelist - ERROR: fv_decomp_set failed to find a decomposition.
npr_yz needs to be set by the user. at /scratch/x1919a03/Models/cesm2_0_1/components/cam/bld/build-namelist line 4409."
The model builds fine with NTASKS=1020. I wanted to increase NTASKS to make the simulations faster. Is there any formula to set NTASKS violating which giving me the aforementioned error? Please suggest. Thanks.

 

[eaton]

The f09 grid has 192 latitudes and FV requires 3 latitudes per subdomain. So you get a max of 64 subdomains in latitude. With 32 levels the biggest decomp you can set would be npr_yz=64,32,32,64. This translates to 64*32 or 2048 tasks. The function fv_decomp_set in the build-namelist script has further description.

 

[bidyut]

Thanks for the clarification Eaton. I was trying to understand the comments in the "sub fv_decomp_set" ... your reply made it clear to me. It's working with NTASKS=2048 now. Many thanks.