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Subject: Re: [OMPI devel] RFC: sm Latency
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2009-01-20 20:56:53

On Jan 20, 2009, at 8:53 PM, Jeff Squyres wrote:

> This all sounds really great to me. I agree with most of what has
> been said -- e.g., benchmarks *are* important. Improving them can
> even sometimes have the side effect of improving real
> applications. ;-)
> My one big concern is the moving of architectural boundaries of
> making the btl understand MPI match headers. But even there, I'm
> torn:
> 1. I understand why it is better -- performance-wise -- to do this.
> And the performance improvement results are hard to argue with. We
> took a similar approach with ORTE; ORTE is now OMPI-specific, and
> many, many things have become better (from the OMPI perspective, at
> least).
> 2. We all have the knee-jerk reaction that we don't want to have the
> BTLs know anything about MPI semantics because they've always been
> that way and it has been a useful abstraction barrier. Now there's
> even a project afoot to move the BTLs out into a separate later that
> cannot know about MPI (so that other things can be built upon it).
> But are we sacrificing potential MPI performance here? I think
> that's one important question.
> Eugene: you mentioned that there are other possibilities to having
> the BTL understand match headers, such as a callback into the PML.
> Have you tried this approach to see what the performance cost would
> be, perchance?

I neglected to say: the point of asking this question is an attempt to
quantify the performance gain of having the BTL understand the match
header. Specifically: is it a noticeable/important performance gain
to have change our age-old abstraction barrier? Or is another
approach just as good, performance-wise?

Jeff Squyres
Cisco Systems