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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Rick Otten <rottenwindfish@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Queries intermittently slow
Date: Thu, 07 Jan 2016 13:34:51 -0500
Message-ID: <13016.1452191691@sss.pgh.pa.us> (raw)
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Rick Otten <rottenwindfish@gmail.com> writes:
> On Thu, Jan 7, 2016 at 12:51 PM, Scott Rankin <srankin@motus.com> wrote:
>> Winner!  Both of those settings were set to always, and as soon as I
>> turned them off, the query times smoothed right out.

> Is this generally true for all PostgreSQL systems on Linux, or only for
> specific use cases?

It's fairly well established that the implementation of transparent
huge pages in Linux kernels from the 2.6-or-so era sucks, and you're
best off turning it off if you care about consistency of performance.
I am not sure whether modern kernels have improved this area.

I think you can get an idea of how big a problem you have by noting
the accumulated runtime of the khugepaged daemon.

(BTW, it would likely be a good thing to collect some current wisdom
in this area and add it to section 17.4 of our docs.)

			regards, tom lane


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