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Subject: Re: [OMPI devel] SM init failures
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2009-03-30 14:42:07


Jeff Squyres wrote:

> It's half done, actually. But it was still going to be an option,
> not necessarily the only way to do it:
> http://www.open-mpi.org/hg/hgwebdir.cgi/jsquyres/shm-sysv/
>
> On Mar 30, 2009, at 1:40 PM, Tim Mattox wrote:
>
>> I've been lurking on this conversation, and I am again left with the
>> impression
>> that the underlying shared memory configuration based on sharing a file
>> is flawed. Why not use a System V shared memory segment without a
>> backing file as I described in ticket #1320?
>
In some performance experiments so far, it appears that the start-up
performance of the two approaches are rather different. I don't
understand the results, but I note here that they're different.

Touching all that shared memory upon startup (which, as I noted, we
essentially do now anyhow) adds noticeably to the start-up time. (Less
so now with the single-queue changes. Whether the time is tolerable or
not is a matter of taste, etc.) Going from mmap to SysV speeds that
time up a lot in the tests I've done. But then (mysteriously) the time
to exchange first messages takes much longer.

I'm baffled. For now, I just point out that the performance
characteristics could be different.