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Subject: Re: [OMPI devel] Hang in collectives involving shared memory
From: Ashley Pittman (ashley_at_[hidden])
Date: 2009-06-10 10:51:41


Ralph,

If I may say this is exactly the type of problem the tool I have been
working on recently aims to help with and I'd be happy to help you
through it.

Firstly I'd say of the three collectives you mention, MPI_Allgather,
MPI_Reduce and MPI_Bcast one exhibit a many-to-many, one a many-to-one
and the last a many-to-one communication pattern. The scenario of a
root process falling behind and getting swamped in comms is a plausible
one for MPI_Reduce only but doesn't hold water with the other two. You
also don't mention if the loop is over a single collective or if you
have loop calling a number of different collectives each iteration.

padb, the tool I've been working on has the ability to look at parallel
jobs and report on the state of collective comms and should help narrow
you down on erroneous processes and those simply blocked waiting for
comms. I'd recommend using it to look at maybe four or five instances
where the application has hung and look for any common features between
them.

Let me know if you are willing to try this route and I'll talk, the code
is downloadable from http://padb.pittman.org.uk and if you want the full
collective functionality you'll need to patch openmp with the patch from
http://padb.pittman.org.uk/extensions.html

Ashley,

-- 
Ashley Pittman, Bath, UK.
Padb - A parallel job inspection tool for cluster computing
http://padb.pittman.org.uk