From: | Josh Berkus <josh(at)agliodbs(dot)com> |
---|---|
To: | Kevin Grittner <kgrittn(at)ymail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Bruce Momjian <bruce(at)momjian(dot)us>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Auto-tuning work_mem and maintenance_work_mem |
Date: | 2014-03-10 23:28:01 |
Message-ID: | 531E4A81.9020903@agliodbs.com |
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Lists: | pgsql-hackers |
On 03/10/2014 03:16 PM, Kevin Grittner wrote:
> I only have anecdotal evidence, though. I have seen it help dozens
> of times, and have yet to see it hurt. That said, most people on
> this list are probably capable of engineering a benchmark which
> will show whichever result they would prefer. I would prefer to
> hear about other data points based on field experience with
> production systems. I haven't offered the trivial patch because
> when I've raised the point before, there didn't seem to be anyone
> else who had the same experience. It's good to hear that Andres
> has seen this, too.
The problem with cpu_tuple_cost is that it's used in several places by
the planner and makes it hard to model what the effect of any change
would be. If we had a good general benchmark which actually gave the
query planner a workout, we could come up with some reasonable default
settings, but right now we can't.
Back in 2004-2006 era, when CPU speeds had leapfrogged ahead of disk
speeds (which were largely unchanged from 2000), I was routinely
*lowering* cpu_tuple_cost (and cpu_index_tuple_cost) to get better
plans. This was baked into early versions of Greenplum for that reason.
So I'm not saying that we shouldn't change the default for
cpu_tuple_cost. I'm saying that we currently don't have enough
information on *when* and *how much* to change it.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com
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