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Subject: Re: [OMPI users] BTL layer
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-09-22 07:38:21

On Sep 22, 2010, at 3:46 AM, Gabriele Fatigati wrote:

> i'm tuning collectives of OpenMPI 1.4.2 with OTPO. I have a little question about BTL. This layer is involves just in point-to-point communication or also in collectives routines?
> Because i've noted that changing some blt parameters like btl_sm_eager_limit and doing one collective routine, performances can have very different behaviour.

The BTLs are dumb byte transport layers -- they have no idea of any MPI semantics.

But that being said, they act as the lowest layers for the MPI point-to-point functions (MPI_SEND and the like). They may *also* act as the lowest layers for all of the collective functions. Some collective algorithms call MPI_SEND to do their individual sends, for example. Others may invoke BTL send/receive functions directly. Others don't use the BTLs at all.

Jeff Squyres
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