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

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Message-ID: <20081118064943.332516f7@infradead.org>
Date:	Tue, 18 Nov 2008 06:49:43 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
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 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)

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.

-- 
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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