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Re: Posix Shared Mem patch

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Josh Berkus <josh(at)agliodbs(dot)com>
Subject: Re: Posix Shared Mem patch
Date: 2012-07-03 15:42:29
Message-ID: CA+TgmoZ=sEN5R=6n4spO6HDzGUpO8=HEKG8S+BYqO6Phn7Hx0g@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Jul 3, 2012 at 11:36 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> Btw, RhodiumToad/Andrew Gierth on irc talked about a reason why sysv shared
> memory might be advantageous on some platforms. E.g. on freebsd there is the
> kern.ipc.shm_use_phys setting which prevents paging out shared memory and also
> seems to make tlb translation cheaper. There does not seem to exist an
> alternative for anonymous mmap.
> So maybe we should make that a config option?

Yeah, I was noticing some notes to that effect in the documentation
this morning.  I think the alternative for anonymous mmap is mlock().
However, that can hit kernel limits of its own.  I'm not sure what the
best thing to do about this is.  I think most users will want mlock...
but maybe not all?  So we end up with one option for whether to use
mlock and another for whether to use more or less System V shm?
Sounds confusing.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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