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Subject: Re: [OMPI users] System hang-up on MPI_Reduce
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-11-10 16:57:31

Yeah, that is "normal". It has to do with unexpected messages.

When you have procs running at significantly different speeds, the
various operations get far enough out of sync that the memory consumed
by recvd messages not yet processed grows too large.

Instead of sticking barriers into your code, you can have OMPI do an
internal sync after every so many operations to avoid the problem.
This is done by enabling the "sync" collective component, and then
adjusting the number of operations between forced syncs.

Do an "ompi_info --params coll sync" to see the options. Then set the
coll_sync_priority to something like 100 and it should work for you.


On Nov 10, 2009, at 2:45 PM, Glembek Ondřej wrote:

> Hi,
> I am using MPI_Reduce operation on 122880x400 matrix of doubles. The
> parallel job runs on 32 machines, each having different processor in
> terms of speed, but the architecture and OS is the same on all
> machines (x86_64). The task is a typical map-and-reduce, i.e. each
> of the processes collects some data, which is then summed
> (MPI_Reduce w. MPI_SUM).
> Having different processors, each of the jobs comes to the
> MPI_Reduce in different time.
> The *first problem* came when I called MPI_Reduce on the whole
> matrix. The system ended up with *MPI_ERR_OTHER error*, each time on
> different rank. I fixed this problem by chunking up the matrix into
> 2048 submatrices, calling MPI_Reduce in cycle.
> However *second problem* arose --- MPI_Reduce hangs up... It
> apparently gets stuck in some kind of dead-lock or something like
> that. It seems that if the processors are of similar speed, the
> problem disappears, however I cannot provide this condition all the
> time.
> I managed to get rid of the problem (at least after few non-
> problematic iterations) by sticking MPI_Barrier before the
> MPI_Reduce line.
> The questions are:
> 1) is this a usual behavior???
> 2) is there some kind of timeout for MPI_Reduce???
> 3) why does MPI_Reduce die on large amount of data if the system has
> enough address space (64 bit compilation)
> Thanx
> Ondrej Glembek
> --
> Ondrej Glembek, PhD student E-mail: glembek_at_[hidden]
> UPGM FIT VUT Brno, L226 Web:
> Bozetechova 2, 612 66 Phone: +420 54114-1292
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