linux-kernel - Re: Active waiting with yield()

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Message-ID: <Pine.LNX.4.64.0811181211360.13418@hs20-bc2-1.build.redhat.com>
Date:	Tue, 18 Nov 2008 12:14:53 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, rml@...h9.net,
	Alasdair G Kergon <agk@...hat.com>,
	Milan Broz <mbroz@...hat.com>
Subject: Re: Active waiting with yield()

On Tue, 18 Nov 2008, Arjan van de Ven wrote:

> On Tue, 18 Nov 2008 09:34:16 -0500 (EST)
> > So what are the reasons why you (and others) are against active
> > waiting? All you are saying is that my reasons are wrong, but you
> > haven't single example when active waiting causes trouble. If there
> > is a workload when waiting 1ms-to-10ms with mdelay(1) on driver
> > unload would cause discomfort to the user, describe it.
> > 
> 
> mdelay()
> * costs you quite a bit of power
> * will cause your cpu to go to full speed
> * makes it more likely that your fan goes on
> * takes away CPU time from others who do want to run
>   - including the guy you are waiting for!
> * if you do it with interrupts off you can even cause time skew
> * adds 10 milliseconds of latency to the entire system, which is very
>   user noticable in a desktop environment (the threshold for that is
>   like 1 or 2 milliseconds total)

msleep(1) should be better, mdelay doesn't give other processes a chance 
to run.

> now there are some cases, mostly during error recovery or driver init
> slowpaths where mdelay() can be justified, but "I'm too lazy to use a
> waitqueue or other sleeping construct" is not one of them.

That is exactly what my initial post was about. I agree that using polling 
on normal request processing is stupid, but I don't see why some people 
don't like msleep() it even in slow paths (such as driver unload).

Mikulas

> -- 
> 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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