linux-kernel - Re: [PATCH] cpufreq fix timer teardown in conservative governor (2.6.30-rc2)

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Date:	Sun, 26 Apr 2009 12:31:39 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	Len Brown <lenb@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, gregkh@...e.de,
	stable@...nel.org, cpufreq@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, rjw@...k.pl,
	Ben Slusky <sluskyb@...anoiacs.org>,
	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq fix timer teardown in conservative governor
	(2.6.30-rc2)

* Henrique de Moraes Holschuh (hmh@....eng.br) wrote:
> On Fri, 24 Apr 2009, Mathieu Desnoyers wrote:
> > * Len Brown (lenb@...nel.org) wrote:
> > > Somebody please remind me why we are spending effort to 
> > > maintain the conservative governor instead of deleting it.
> > 
> > Documentation/cpu-freq/governors.txt
> > 
> > "The CPUfreq governor "conservative", much like the "ondemand"
> > governor, sets the CPU depending on the current usage.  It differs in
> > behaviour in that it gracefully increases and decreases the CPU speed
> > rather than jumping to max speed the moment there is any load on the
> > CPU.  This behaviour more suitable in a battery powered environment."
> > 
> > So better battery usage seems to be the reason why conservative lives.
> 
> Yeah, but the question is: is it really better in practice? race-to-idle
> works better with ondemand.  Note: that needs to be answered not just for
> the current crop of mobile processors, but also for at least stuff as old as
> the Pentium M and Pentium 4 M.
> 
> What it _does_ help, is in broken !@#$ hardware that makes a lot of noise
> due to "singing capacitors" if you use ondemand (because conservative will
> make less noise as it causes more smooth transitions).  NOHZ helped a great
> deal there, too.
> 

I effectively have such singing capacitors on my laptop, and I still
have good ears.

> I don't know if there are battery environments where a harsher work profile
> by the CPU are a bad idea.  If there are any, conservative will also help
> there.
> 

Good question. We might also consider that anyway code duplication
between ondemand and conservative is a bad thing. Those look so similar
that part of them should probably be merged, with a sysfs flag and
kernel parameter to select the behavior.

Mathieu


> -- 
>   "One disk to rule them all, One disk to find them. One disk to bring
>   them all and in the darkness grind them. In the Land of Redmond
>   where the shadows lie." -- The Silicon Valley Tarot
>   Henrique Holschuh

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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