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Subject: Re: [OMPI users] MPI_Bcast issue
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-08-09 10:07:45


Personally, I've been having trouble following the explanations of the problem.  Perhaps it'd be helpful if you gave us an example of how to reproduce the problem.  E.g., short sample code and how you run the example to produce the problem.  The shorter the example, the greater the odds of resolution.

From: Randolph Pullen <randolph_pullen@yahoo.com.au>
To: users@open-mpi.org
Date: 08/07/2010 01:23 AM
Subject: [OMPI users] MPI_Bcast issue
Sent by: users-bounces@open-mpi.org

I seem to be having a problem with MPI_Bcast.
My massive I/O intensive data movement program must broadcast from n to n nodes. My problem starts because I require 2 processes per node, a sender and a receiver and I have implemented these using MPI processes rather than tackle the complexities of threads on MPI.

Consequently, broadcast and calls like alltoall are not completely helpful.  The dataset is huge and each node must end up with a complete copy built by the large number of contributing broadcasts from the sending nodes.  Network efficiency and run time are paramount.

As I don’t want to needlessly broadcast all this data to the sending nodes and I have a perfectly good MPI program that distributes globally from a single node (1 to N), I took the unusual decision to start N copies of this program by spawning the MPI system from the PVM system in an effort to get my N to N concurrent transfers.

It seems that the broadcasts running on concurrent MPI environments collide and cause all but the first process to hang waiting for their broadcasts.  This theory seems to be confirmed by introducing a sleep of n-1 seconds before the first MPI_Bcast  call on each node, which results in the code working perfectly.  (total run time 55 seconds, 3 nodes, standard TCP stack)

My guess is that unlike PVM, OpenMPI implements broadcasts with broadcasts rather than multicasts.  Can someone confirm this?  Is this a bug?

Is there any multicast or N to N broadcast where sender processes can avoid participating when they don’t need to?