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Subject: Re: [OMPI users] round-robin scheduling question [hostfile]
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-02-20 10:19:48

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/


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?) [
> ], 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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