It is a little bit of both:
* historical, because most MPI's default to mapping by slot, and
* performance, because procs that share a node can communicate via
shared memory, which is faster than sending messages over an
interconnect, and most apps are communication-bound
If your app is disk-intensive, then mapping it -bynode may be a better
option for you. That's why we provide it. Note, however, that you can
still wind up with multiple procs on a node. All "bynode" means is
that the ranks are numbered consecutively bynode - it doesn't mean
that there is only one proc/node.
If you truly want one proc/node, then you should use the -pernode
option. This maps one proc on each node up to either the number of
procs you specified or the number of available nodes. If you don't
specify -np, we just put one proc on each node in your allocation/
hostfile.
HTH
Ralph
On Feb 20, 2009, at 1:25 AM, Raymond Wan wrote:
Hi all,
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...
Thank you!
Ray
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