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Subject: Re: [OMPI users] openmpi credits for eager messages
From: George Bosilca (bosilca_at_[hidden])
Date: 2008-02-01 17:07:45


The Voltaire tech was right. There is no credit management at the
upper level, each BTL is allowed to do it (if needed). This doesn't
means the transport is not reliable. Most of the devices have internal
flow control, and Open MPI rely on it instead of implementing our own.
However, the devices that do not provide in their low level drivers or
hardware such feature, have it implemented at the BTL layer. As an
example, infiniband have a flow control mechanism implemented in the
BTL.

   george.

On Feb 1, 2008, at 3:05 PM, Sacerdoti, Federico wrote:

> Hi,
>
> I am readying an openmpi 1.2.5 software stack for use with a
> many-thousand core cluster. I have a question about sending small
> messages that I hope can be answered on this list.
>
> I was under the impression that if node A wants to send a small MPI
> message to node B, it must have a credit to do so. The credit
> assures A
> that B has enough buffer space to accept the message. Credits are
> required by the mpi layer regardless of the BTL transport layer used.
>
> I have been told by a Voltaire tech that this is not so, the credits
> are
> used by the infiniband transport layer to reliably send a message, and
> is not an openmpi feature.
>
> Thanks,
> Federico
>
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