Set mpirun to None when a single MPI task is being used (similar to handling of mpi-serial)

[ekluzek]

mpi-serial used to be required for building serially. It no longer is for modern MPI libraries. One of the benefits of mpi-serial is that it removes the complexity of having to use a mpirun command and options which is both different on different machines, adds complexity, makes it harder to port to a machine for a serial case, and often means you can't run a simple case on the command line.

There is special handling in CIME for mpi-serial that allows mpirun to be None. For this I think similar logic can be used to set it to None, in env_mach_specific.py in _find_best_mpirun_match.

The other way to handle this would be to add a setting for NTASKS==1 to config_machine.xml for the specific machines we want to do this for.

[ekluzek]

Doing this allows users that are running serially (for example with CTSM single point tower sites such as NEON) to run short cases on personal machines. And not having to figure the complexity of mpirun options makes it easier for users on these machines to get serial cases working.

./case.submit --no-batch

without the change to mpirun the above won't work on machines that don't allow mpirun to be used outside of batch submission.

NOTE: On Derecho, do NOT run on login nodes with above (it'll log you off if you do too much of above). But, you can use qcmd with above as well.

[jedwards4b]

@ekluzek I don't think we should encourage running on the login nodes. Use qcmd or run on casper.

[ekluzek]

Good point @jedwards4b I've edited my above comment to emphasize the use on non-HPC machines. And give a specific warning for Derecho. Let me know if you have any further suggestions...

[ekluzek]

Note, the ability to run cases like this with

./case.submit --no-batch

is important for one user who has an unusual use-case. He runs a bunch of cases all at once (multi-instance doesn't work for him because the cases have differences that multi-instance doesn't allow), so it's under batch, but he needs the above to get his setup to work.

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[github-actions]

This issue was closed because it has been stalled for 5 days with no activity.