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Re: Posix Shared Mem patch

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
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 18:00:06
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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.

			regards, tom lane

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