Thanks to Ralph, Gus and Georg for your input!
I was diverted to other things for a week, but now back on track...
You deserve to have the question marks straightened out first: The two
application are (1) a commercial solver and (2) an in-house code handling
some special physics. The two codes will run sequentially, taking turns
one time step at a time. The commercial solver runs one time step, outputs
some data to file, then waits for the in-house code to do some work, then
goes to next time step. The in-house code is actually restarted each time
it has work to do. (This is what I mean with "loosely coupled".)
As the two codes never work at the same time, I would like them to use the
same hardware. The commercial solver may be reluctant to release its cores
to other processes ("agressive"), but I hope it will at the end of time
steps... The commercial code will just be started as usual, using the full
allocation of the MOAB job. The in-house code is the one using openmpi,
and I want it to use all of the cores in the first node of the allocation,
and only those.
As Ralph suggests, it seems very convenient to use the -host option with
relative node syntax. I also found some other references on how mpirun
handles host info from the resources managers. Starting the two codes as
background jobs, like Georg does, sounds good. I will simply give it a
spin and see how it works...
Thanks again for your time,
Ola
