Date: Mon, 14 Aug 2017 09:36:10 -0700
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Thomas Munro <thomas.munro@enterprisedb.com>,
	Dilip Kumar <dilipbalaut@gmail.com>,
	Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: shared memory based stat collector (was: Sharing
 record typmods between backends)
Message-ID: <20170814163610.pgdzwygma2q6csi6@alap3.anarazel.de>
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On 2017-08-14 12:28:39 -0400, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > On Mon, Aug 14, 2017 at 12:16 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> Just FYI, the only values being reported by buildfarm animals are
> >> "posix", "sysv", and "windows".  So while mmap may be a thing,
> >> it's an untested thing.
> 
> > I'm pretty sure I dev-tested it before committing anything, but,
> > certainly, having ongoing BF coverage woudn't be a bad thing.
> 
> Looking closer, the reason those are the only reported values is
> that those are the only possible results from initdb's
> choose_dsm_implementation().  So the real question here is whether
> "mmap" should be considered to dominate "sysv" if it's available.

No mmap isn't a good option - it's file backed mmap, rather than
anonymous mmap. To my knowledge there's no good portable way to use
anonymous mmap to share memory across processes unless established
before a fork().


> If so, why isn't choose_dsm_implementation() trying it; and if not,
> why are we carrying it?

I think the idea was that there might be platforms that require it, but
...


Greetings,

Andres Freund


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