Subject: Re: Linux I/O tuning:  CFQ vs. deadline
From: Jeff Davis <pgsql@j-davis.com>
To: Josh Berkus <josh@agliodbs.com>
Cc: Greg Smith <greg@2ndquadrant.com>,
	Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	pgsql-performance@postgresql.org, Albe Laurenz <laurenz.albe@wien.gv.at>
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	<4B702E05.1000708@2ndquadrant.com>  <4B704EA0.3050906@agliodbs.com>
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Date: Tue, 16 Feb 2010 13:34:11 -0800
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On Mon, 2010-02-08 at 09:49 -0800, Josh Berkus wrote:
> FWIW, back when deadline was first introduced Mark Wong did some tests
> and found Deadline to be the fastest of 4 on DBT2 ... but only by about
> 5%.  If the read vs. checkpoint analysis is correct, what was happening
> is the penalty for checkpoints on deadline was almost wiping out the
> advantage for reads, but not quite.

I also did some tests when I was putting together my Synchronized Scan
benchmarks:

http://j-davis.com/postgresql/83v82_scans.html

CFQ was so slow that I didn't include it in the results at all.

The tests weren't intended to compare schedulers, so I did most of the
tests with anticipatory (at least the ones on linux; I also tested
freebsd). However, I have some raw data from the tests I did run with
CFQ:

http://j-davis.com/postgresql/results/

They will take some interpretation (again, not intended as scheduler
benchmarks). The server was modified to record a log message every N
page accesses.

Regards,
	Jeff Davis