linux-kernel - Re: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO

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Date:	Fri, 23 Apr 2010 07:24:39 +0200
From:	Pavel Machek <pavel@....cz>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, peterz@...radead.org, tglx@...utronix.de,
	davej@...hat.com, cpufreq@...r.kernel.org
Subject: Re: [PATCH 7/7] ondemand: Solve the big performance issue with
 ondemand during disk IO

Hi!

> The ondemand cpufreq governor uses CPU busy time (e.g. not-idle time) as
> a measure for scaling the CPU frequency up or down.
> If the CPU is busy, the CPU frequency scales up, if it's idle, the CPU
> frequency scales down. Effectively, it uses the CPU busy time as proxy
> variable for the more nebulous "how critical is performance right now"
> question.
> 
> This algorithm falls flat on its face in the light of workloads where
> you're alternatingly disk and CPU bound, such as the ever popular
> "git grep", but also things like startup of programs and maildir using
> email clients... much to the chagarin of Andrew Morton.
> 
> This patch changes the ondemand algorithm to count iowait time as busy,
> not idle, time. As shown in the breakdown cases above, iowait is performance
> critical often, and by counting iowait, the proxy variable becomes a more
> accurate representation of the "how critical is performance"
> question.

Well, and now, if you do something like cat /dev/<your usb1.1 hdd> >
/dev/null, you'll keep cpu on max frequency. Not a problem for new
core i7, but probably big deal for athlon 64.

Maybe modern cpus can and should simply react faster?

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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