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

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Message-ID: <201004191629.39339.tvrtko.ursulin@sophos.com>
Date:	Mon, 19 Apr 2010 16:29:39 +0100
From:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	"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 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?

> > One idea I had but a) never had time to implement it and b) was not
> > sure it would be accepted anyway, was to modify ondemand governor to
> > ramp up instantly, but slow down slowly (in a configurable way).
>
> that's what ondemand does already.

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.

Tvrtko

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