According to FAQ 14 (How do I control how my processes are scheduled across nodes?) [http://www.open-mpi.org/faq/?category=running#mpirun-scheduling], it says that the default scheduling policy is by slot and not by node. I'm curious why the default is "by slot" since I am thinking of explicitly specifying by node but I'm wondering if there is an issue which I haven't considered.
I would think that one reason for "by node" is to distribute HDD access across machines [as is the case for me since my program is HDD access intensive]. Or perhaps I am mistaken? I'm now thinking that "by slot" is the default because processes with ranks that are close together might do similar tasks and you would want them on the same node? Is that the reason?
Also, at the end of this FAQ, it says "NOTE: This is the scheduling policy in Open MPI because of a long historical precendent..." -- does this "This" refer to "the fact that there are two scheduling policies" or "the fact that 'by slot' is the default"? If the latter, then that explains why "by slot" is the default, I guess...