Date: Mon, 4 Nov 2013 12:19:12 +0100
From: Andres Freund <andres@2ndquadrant.com>
To: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>,
 Tom Lane <tgl@sss.pgh.pa.us>, Andrew Dunstan <andrew@dunslane.net>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Something fishy happening on frogmouth
Message-ID: <20131104111912.GD17233@awork2.anarazel.de>
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On 2013-11-04 13:13:27 +0200, Heikki Linnakangas wrote:
> On 04.11.2013 11:55, Andres Freund wrote:
> >On 2013-11-04 10:27:47 +0200, Heikki Linnakangas wrote:
> >>Postmaster creates the POSIX shared memory object at startup, by calling
> >>shm_open(), and immediately calls shm_unlink on it. That way, once all the
> >>processes have exited, the object will be removed automatically. Child
> >>processes inherit the file descriptor at fork(), and don't need to call
> >>shm_open, just mmap().
> >
> >Uh. Won't that completely and utterly remove the point of dsm which is
> >that you can create segments *after* startup? We surely don't want to
> >start overallocating enough shmem so we don't ever dynamically need to
> >allocate segments.

> You don't need to allocate the shared memory beforehand, only create the
> file descriptor. Note that the size of the segment is specified in the
> shm_open() call, but the mmap() that comes later.
>
> If we need a large amount of small segments, so that it's not feasible to
> shm_open() them all at postmaster startup, you could shm_open() just one
> segment, and carve out smaller segments from it by mmap()ing at different
> offsets.

That quickly will result in fragmentation which we don't have the tools
to handle.

> >Also, I don't think it's portable across platforms to access segments
> >that already have been unlinked.
> 
> See
> http://pubs.opengroup.org/onlinepubs/009695299/functions/shm_unlink.html:
> "If one or more references to the shared memory object exist when the object
> is unlinked, the name shall be removed before shm_unlink() returns, but the
> removal of the memory object contents shall be postponed until all open and
> map references to the shared memory object have been removed."

We also support sysv shmem and have the same cleanup problem there.

> That doesn't explicitly say that a new shm_open() on the file descriptor
> must still work after shm_unlink, but I would be surprised if there is a
> platform where it doesn't.

Probably true.

> >I think this is looking for a solution without an actually relevant
> >problem disregarding the actual problem space.

To make that clearer: I think the discussions about making it impossible
to leak segments after rm -rf are the irrelevant problem.

> I agree. What are these dynamic shared memory segments supposed to be used
> for?

Parallel sort and stuff like that.

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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