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

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Date:	Mon, 26 Apr 2010 17:30:31 -0400
From:	Rik van Riel <riel@...hat.com>
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

On 04/18/2010 03:03 PM, Arjan van de Ven wrote:
>  From 27966bedabea83c4f3ae77507eceb746b1f6ebae Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven<arjan@...ux.intel.com>
> Date: Sun, 18 Apr 2010 11:15:56 -0700
> Subject: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO
>
> 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.
>
> The problem and fix are both verified with the "perf timechar" tool.
>
> Signed-off-by: Arjan van de Ven<arjan@...ux.intel.com>

Reviewed-by: Rik van Riel <riel@...hat.com>

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