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 <r...@lanl.gov> 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
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Tim Mattox, Ph.D. - http://homepage.mac.com/tmattox/
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