Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Magnus Hagander <magnus(at)hagander(dot)net>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Claudio Freire <klaussfreire(at)gmail(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers
Date: 2014-05-15 12:06:04
Message-ID: 20140515120604.GA28532@momjian.us
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On Tue, May 6, 2014 at 11:15:17PM +0100, Simon Riggs wrote:
> > Well, for what it's worth, I've encountered systems where setting
> > effective_cache_size too low resulted in bad query plans, but I've
> > never encountered the reverse situation.
>
> I agree with that.
>
> Though that misses my point, which is that you can't know that all of
> that memory is truly available on a server with many concurrent users.
> Choosing settings that undercost memory intensive plans are not the
> best choice for a default strategy in a mixed workload when cache may
> be better used elsewhere, even if such settings make sense for some
> individual users.

This is the same problem we had with auto-tuning work_mem, in that we
didn't know what other concurrent activity was happening. Seems we need
concurrent activity detection before auto-tuning work_mem and
effective_cache_size.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +

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