linux-kernel - Re: [PATCH v3 00/18] periodic write-back timer optimization

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Message-ID: <4A55EAF9.9070907@nokia.com>
Date:	Thu, 09 Jul 2009 16:04:57 +0300
From:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	Jens Axboe <jens.axboe@...cle.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 00/18] periodic write-back timer optimization

Andi Kleen wrote:
> Artem Bityutskiy <Artem.Bityutskiy@...ia.com> writes:
> 
>> The patches attempt to optimize the periodic write-back and stop it when
>> there are no dirty data. IOW, we do not want the thread to wake up every
>> 5 seconds (by default), find there is nothing to do, and so on.
> 
> Is waking up every 5 seconds really a problem?

Yes, it is. In OMAP3 we may switch the core and most of peripherals
to off and we may stay in off and consume really few power. And waking
up from off every 5 seconds for no good reason is bad.

> The normal rule of thumb is iirc that longer sleep times than a few hundred
> ms give dimishing returns in terms of power saving.

I think our pm guys measured this on OMAP3 and it translates to
~hour of idle mode. And yes, this is bad for devices which
run from battery.

> A simple way might be simple to batch the timer better with other timers.

I agree. There is a lot of work in this direction. There are many
places where we could use range hrtimers or deferrable timers
and improve PM. But I consider this to be the second level of
optimization. The first level is to get rid of unneeded events
completely.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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