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

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Message-ID: <20081117180149.7a48d2e9@lxorguk.ukuu.org.uk>
Date:	Mon, 17 Nov 2008 18:01:49 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	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()

> --- so if the driver processes more than 100000 requests between reboots, 
> wait queues actually slow things down.

Versus power consumption and virtualisation considerations. Plus your
numbers are wrong. You seem terribly keen to ignore the fact that the
true cost is a predicted branch and usually a predicted branch of a
cached variable and you'll only touch the wait queue in rare cases.

I'd also note as an aside modern drivers usually run off krefs so
destruction and thus closedown is refcounted and comes off the last kref
destruct.

Alan
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