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Subject: Re: [OMPI users] openmpi credits for eager messages
From: Gleb Natapov (glebn_at_[hidden])
Date: 2008-02-05 08:29:21

On Tue, Feb 05, 2008 at 08:07:59AM -0500, Richard Treumann wrote:
> There is no misunderstanding of the MPI standard or the definition of
> blocking in the bug3 example. Both bug 3 and the example I provided are
> valid MPI.
> As you say, blocking means the send buffer can be reused when the MPI_Send
> returns. This is exactly what bug3 is count on.
> MPI is a reliable protocol which means that once MPI_Send returns, the
> application can assume the message will be delivered once a matching recv
> is posted. There are only two ways I can think of for MPI to keep that
> guarantee.
> 1) Before return from MPI_Send, copy the envelop and data to some buffer
> that will be preserved until the MPI_Recv gets posted
> 2) delay the return from MPI_Send until the MPI_Recv is posted and then
> move data from the intact send buffer to the posted receive buffer. Return
> from MPI_Send
> The requirement in the standard is that if libmpi takes option 1, the
> return from MPI_Send cannot occur unless there is certainty the buffer
> space exists. Libmpi cannot throw the message over the wall and fail the
> job if it cannit be buffered.
As I said Open MPI has flow control on transport layer to prevent messages
from been dropped by network. This mechanism should allow program like
yours to work, but bug3 is another story because it generate huge
amount of unexpected messages and Open MPI has no mechanism to prevent
unexpected messages to blow memory consumption. Your point is that
according to MPI spec this is not a valid behaviour. I am not going to
argue with that especially as you can get desired behaviour by setting
eager limit to zero.

> users-bounces_at_[hidden] wrote on 02/05/2008 02:28:27 AM:
> > On Mon, Feb 04, 2008 at 04:23:13PM -0500, Sacerdoti, Federico wrote:
> > > Bug3 is a test-case derived from a real, scalable application (desmond
> > > for molecular dynamics) that several experienced MPI developers have
> > > worked on. Note the MPI_Send calls of processes N>0 are *blocking*; the
> > > openmpi silently sends them in the background and overwhelms process 0
> > > due to lack of flow control.
> > MPI_Send is *blocking* in MPI sense of the word i.e when MPI_Send returns
> > send buffer can be reused. MPI spec section 3.4.
> >
> > --
> > Gleb.
> > _______________________________________________
> > users mailing list
> > users_at_[hidden]
> >
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