Re: [RFC][PATCH] Cross Memory Attach v2 (resend)

[Robin Holt]

On Tue, Nov 23, 2010 at 11:05:46AM +0100, Brice Goglin wrote:
> Le 23/11/2010 10:25, Christopher Yeoh a écrit :
> > On Mon, 22 Nov 2010 13:05:27 -0800
> > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >   
> >> We have a bit of a track record of adding cool-looking syscalls and
> >> then regretting it a few years later.  Few people use them, and maybe
> >> they weren't so cool after all, and we have to maintain them for
> >> ever. Bugs (sometimes security-relevant ones) remain undiscovered for
> >> long periods because few people use (or care about) the code.
> >>
> >> So I think the bar is a high one - higher than it used to be.
> >> Convince us that this feature is so important that it's worth all
> >> that overhead and risk?
> >>     
> > Well there are the benchmark results to show that there is
> > real improvement for MPI implementations (well at least for those
> > benchmarks ;-) There's also been a few papers written on something
> > quite similar (KNEM) which goes into more detail on the potential gains.
> >
> > http://runtime.bordeaux.inria.fr/knem/
> >
> > I've also heard privately that something very similar has been used in
> > at least one device driver to support intranode operations for quite a
> > while
> >   
> 
> Many HPC hardware vendors implemented something like this in their
> custom drivers to avoid going through their network loopback for local
> communication. Even if their loopback is very fast, going to the NIC and
> back to same host isn't really optimal. And I think all of them kept the
> traditional approach (double-copy across a shared-memory buffer) for
> small messages and only switched to this single-copy model for large
> messages (tens or hundreds of kB). CMA and KNEM are "standardizing" all
> this work and making it portable across multiple HPC platform/networks.

SGI used this concept even for single-byte messages both within the same
and across hosts.

Thanks,
Robin
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