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Re: Posix Shared Mem patch

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Magnus Hagander <magnus(at)hagander(dot)net>, pgsql-hackers(at)postgresql(dot)org, Robert Haas <robertmhaas(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>
Subject: Re: Posix Shared Mem patch
Date: 2012-06-28 18:06:18
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thursday, June 28, 2012 08:00:06 PM Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > On Thursday, June 28, 2012 07:43:16 PM Tom Lane wrote:
> >> I think it *would* be a good idea to mlock if we could.  Setting shmem
> >> large enough that it swaps has always been horrible for performance,
> >> and in sysv-land there's no way to prevent that.  But we can't error
> >> out on permissions failure.
> > 
> > Its also a very good method to get into hard to diagnose OOM situations
> > though. Unless the machine is setup very careful and only runs postgres I
> > don't think its acceptable to do that.
> Well, the permissions angle is actually a good thing here.  There is
> pretty much no risk of the mlock succeeding on a box that hasn't been
> specially configured --- and, in most cases, I think you'd need root
> cooperation to raise postgres' RLIMIT_MEMLOCK.  So I think we could try
> to mlock without having any effect for 99% of users.  The 1% who are
> smart enough to raise the rlimit to something suitable would get better,
> or at least more predictable, performance.
The heightened limit might just as well target at another application and be 
setup a bit to widely. I agree that it is useful, but I think it requires its 
own setting, defaulting to off. Especially as there are no experiences with 
running a larger pg instance that way.


Andres, for once the conservative one, Freund

 Andres Freund	         
 PostgreSQL Development, 24x7 Support, Training & Services

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