Re: POSIX shared memory support

The original patch author:

Chris Marcellino <cmarcellino(at)apple(dot)com>

was not CC'ed as part of this email thread. That was a mistake. Chris,
the email thread discussing your patch is here:

http://archives.postgresql.org/pgsql-hackers/2008-03/msg01262.php

Please read the discussion --- the bottom line is that there isn't much
support for the patch. Magnus was able to do the POSIX usage without
relying on shared memory, but I just talked to him via IM and he said it
used a Win32-specific feature that isn't portable to Unix.

I am holding this patch for the next commit fest in hopes you can adjust
it, but if not the patch will be rejected at that time.

---------------------------------------------------------------------------

Stephen Frost wrote:
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> Chris, et al,
>
> (commit-fest consensus discussion)
> * Chris Marcellino wrote:
> > In case you haven't had enough, here is another version of the code
> > to make Postgres use POSIX shared memory. Along with the issues that
> > have already been addressed, this version ensures that orphaned
> > backends are not in the database when restarting Postgres by using a
> > single 1 byte SysV segment to see who is attached to the segment
> > using shmctl/IPC_STAT/nattach.
>
> This really feels like a deal-breaker to me. My first reaction to this
> patch, honestly, is that it's being justified for all the wrong reasons.
> Changing to POSIX shm seems like a reasonable goal in general, provided
> it can do what we need, but doing it to work around silly defaults
> doesn't really work for me. If the real issue you have is with the SysV
> limits then I'd suggest you bring that up with the kernel/distribution
> folks to get them to use something more sane.
>
> Looking around a bit, it looks like it's already being addressed in some
> places, for example Solaris 10 apparently uses 1/4th of memory, while
> Centos 5 uses 4GB. Suse also uses a larger default, from what I
> understand. Supporting this effort to get it raised on various
> platforms and distributions seems like a much better approach.
>
> Additionally, it strikes me that there *is* a limit on POSIX shared
> memory too, generally half of ram on the systems I've looked at, but
> there's no guarentee that'll always be the default or that half of ram
> will always be enough for us. So, even with this change, the problem
> isn't completely 'solved'.
>
> Finding a way for POSIX shm to do what we need, including Tom's
> concerns, without depending on SvsV shm as a crutch work around, would
> make this change much more reasonable and could be justified as moving
> to a well defined POSIX standard, and means we may be able to support
> platforms which either are new and don't implement SysV but just POSIX,
> or cases where SysV is being actively depreceated. Neither of which is
> possible if we're stuck with using it in some cases.
>
> Thanks,
>
> Stephen
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--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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