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Re: Posix Shared Mem patch

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Posix Shared Mem patch
Date: 2012-06-27 13:52:22
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Lists: pgsql-hackers

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Wed, Jun 27, 2012 at 12:00 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Would Posix shmem help with that at all?  Why did you choose not to
> >> use the Posix API, anyway?
> > It seemed more complicated.  If we use the POSIX API, we've got to
> > have code to find a non-colliding name for the shm, and we've got to
> > arrange to clean it up at process exit.  Anonymous shm doesn't require
> > a name and goes away automatically when it's no longer in use.
> I see.  Those are pretty good reasons ...

After talking to Magnus a bit this morning regarding this, it sounds
like what we're doing on Windows is closer to Anonymous shm, except that
they use an intentionally specific name, which also allows them to
detect if any children are still alive by using a "create-if-not-exists"
approach on the shm segment and failing if it still exists.  There were
some corner cases around restarts due to it taking a few seconds for the
Windows kernel to pick up on the fact that all the children are dead and
that the shm segment should go away, but they were able to work around
that, and failure to start is surely much better than possible

What this all boils down to is- can you have a shm segment that goes
away when no one is still attached to it, but actually give it a name
and then detect if it already exists atomically on startup on
Linux/Unixes?  If so, perhaps we could use the same mechanism on both..



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