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Subject: Re: [OMPI devel] Change in hostfile behavior
From: Ralph Castain (rhc_at_[hidden])
Date: 2008-07-28 14:00:06

Actually, this is true today regardless of this change. If two
separate mpirun invocations share a node and attempt to use paffinity,
they will conflict with each other. The problem isn't caused by the
hostfile sub-allocation. The problem is that the two mpiruns have no
knowledge of each other's actions, and hence assign node ranks to each
process independently.

Thus, we would have two procs that think they are node rank=0 and
should therefore bind to the 0 processor, and so on up the line.

Obviously, if you run within one mpirun and have two app_contexts, the
hostfile sub-allocation is fine - mpirun will track node rank across
the app_contexts. It is only the use of multiple mpiruns that share
nodes that causes the problem.

Several of us have discussed this problem and have a proposed solution
for 1.4. Once we get past 1.3 (someday!), we'll bring it to the group.

On Jul 28, 2008, at 10:44 AM, Tim Mattox wrote:

> My only concern is how will this interact with PLPA.
> Say two Open MPI jobs each use "half" the cores (slots) on a
> particular node... how would they be able to bind themselves to
> a disjoint set of cores? I'm not asking you to solve this Ralph, I'm
> just pointing it out so we can maybe warn users that if both jobs
> sharing
> a node try to use processor affinity, we don't make that magically
> work well,
> and that we would expect it to do quite poorly.
> I could see disabling paffinity and/or warning if it was enabled for
> one of these "fractional" nodes.
> On Mon, Jul 28, 2008 at 11:43 AM, Ralph Castain <rhc_at_[hidden]> wrote:
>> Per an earlier telecon, I have modified the hostfile behavior
>> slightly to
>> allow hostfiles to subdivide allocations.
>> Briefly: given an allocation, we allow users to specify --hostfile
>> on a
>> per-app_context basis. In this mode, the hostfile info is used to
>> filter the
>> nodes that will be used for that app_context. However, the prior
>> implementation only filtered the nodes themselves - i.e., it was a
>> binary
>> filter that allowed you to include or exclude an entire node.
>> The change now allows you to include a specified #slots for a given
>> node as
>> opposed to -all- slots from that node. You are limited to the #slots
>> included in the original allocation. I just realized that I hadn't
>> output a
>> warning if you attempt to violate this condition - will do so
>> shortly.
>> Rather than just abort if this happens, I set the allocation to
>> that of the
>> original - please let me know if you would prefer it to abort.
>> If you have interest in this behavior, please check it out and let
>> me know
>> if this meets needs.
>> Ralph
>> _______________________________________________
>> devel mailing list
>> devel_at_[hidden]
> --
> Tim Mattox, Ph.D. -
> tmattox_at_[hidden] || timattox_at_[hidden]
> I'm a bright...
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