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 07:26:02 +0200
From:	Pavel Machek <pavel@....cz>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"davej@...hat.com" <davej@...hat.com>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>
Subject: Re: [PATCH 7/7] ondemand: Solve the big performance issue with
 ondemand during disk IO

On Mon 2010-04-19 17:47:02, Arjan van de Ven wrote:
> On Mon, 19 Apr 2010 16:29:39 +0100
> Tvrtko Ursulin <tvrtko.ursulin@...hos.com> wrote:
> 
> > On Monday 19 Apr 2010 14:46:17 Arjan van de Ven wrote:
> > > On Mon, 19 Apr 2010 10:09:55 +0100
> > > > Or in other words, does a pure IO workload benefit from now higher
> > > > selected frequency?
> > >
> > > no.
> > > Mixed workloads do.
> > > but pure IO workloads also don't suffer since while idle, the
> > > voltage goes down anyway.
> > 
> > You mean that higher frequency does not have effect on power use if
> > CPU is idle? Is that true for all/most processors?
> 
> this is true for most processors that I'm aware of.
> there's exceptions for things like where the idle time is really short,

Is not that exactly what will happen for 'cat /dev/<usb1>' case?

Plus I suspect that older cpus are slower at changing voltages, and
slower at powering down when idle...

> > How and where in the code and how to enable that behaviour? From my
> > experiments frequency goes down to minimum as soon as load goes away.
> > What I was talking about is gradual lowering over a configurable
> > period. It is not power efficient, but it could be good for latency
> > in some workloads.
> 
> it's not even good for that ;-(
> 
> it's better then to stay high longer... at least on modern machines the
> inbetween states are pretty much either useless or actually energy
> hurting compared to the higher state.

So what about hiding those from ondemand on modern hw?
								Pavel

-- 
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