linux-kernel - Re: O_DIRECT question

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Message-ID: <20070206203914.GA4128@ucw.cz>
Date:	Tue, 6 Feb 2007 20:39:15 +0000
From:	Pavel Machek <pavel@....cz>
To:	Denis Vlasenko <vda.linux@...glemail.com>
Cc:	Phillip Susi <psusi@....rr.com>, Michael Tokarev <mjt@....msk.ru>,
	Linus Torvalds <torvalds@...l.org>, Viktor <vvp01@...ox.ru>,
	Aubrey <aubreylee@...il.com>, Hua Zhong <hzhong@...il.com>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	hch@...radead.org, kenneth.w.chen@...karlin.mff.cuni.cz
Subject: Re: O_DIRECT question

Hi!

> > > Which shouldn't be true. There is no fundamental reason why
> > > ordinary writes should be slower than O_DIRECT.
> > 
> > Again, there IS a reason:  O_DIRECT eliminates the cpu overhead of the 
> > kernel-user copy,
> 
> You assume that ordinary read()/write() is *required* to do the copying.
> It doesn't. Kernel is allowed to do direct DMAing in this case too.

Kernel is allowed, but it is practically impossible to code. It would
require slow MMU magic.

							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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