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

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Message-ID: <20100420040708.1ae390b1@infradead.org>
Date:	Tue, 20 Apr 2010 04:07:08 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Thomas Renninger <trenn@...e.de>
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 Tue, 20 Apr 2010 11:29:02 +0200
Thomas Renninger <trenn@...e.de> wrote:

> Have you looked at:
> /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_transition_latency
> and
> /sys/devices/system/cpu/cpu0/cpufreq/ondemand/sampling_rate
> Transition latency comes from ACPI tables and sampling rate depends
> on it.

of course I have; in practice on systems i use it is always a 10
milliseconds interval, which is the minimum it gets.
Yes this was a bug, no it wasn't the bug here ;-)

> Reducing the sampling rate, significantly reduces performance loss

while it does, it is not nearly sufficient for the alternating IO/CPU
cases I've looked at. I've looked at many timecharts for various IO/CPU
workloads, including my normal own use as well as Andrews and the CPU
busy periods are in the 1 - 20 msec range between IOs most of the time,
for which a 10 msec sampling is obviously problematic. 

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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