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Re: Posix Shared Mem patch

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Posix Shared Mem patch
Date: 2012-06-27 11:34:48
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Lists: pgsql-hackers
On Wed, Jun 27, 2012 at 12:00 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> So, here's a patch.  Instead of using POSIX shmem, I just took the
>> expedient of using mmap() to map a block of MAP_SHARED|MAP_ANONYMOUS
>> memory.  The sysv shm is still allocated, but it's just a copy of
>> PGShmemHeader; the "real" shared memory is the anonymous block.  This
>> won't work if EXEC_BACKEND is defined so it just falls back on
>> straight sysv shm in that case.
> Um.  I hadn't thought about the EXEC_BACKEND interaction, but that seems
> like a bit of a showstopper.  I would not like to give up the ability
> to debug EXEC_BACKEND mode on Unixen.
> 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.

With respect to EXEC_BACKEND, I wasn't proposing to kill it, just to
make it continue to use a full-sized sysv shm.

Robert Haas
The Enterprise PostgreSQL Company

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