Thanks Jeff,

and.. what about RDMA? It works only with point-to-point or also with collectives?

2010/9/22 Jeff Squyres <jsquyres@cisco.com>
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
jsquyres@cisco.com
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Ing. Gabriele Fatigati

Parallel programmer

CINECA Systems & Tecnologies Department

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