Re: O_DIRECT question

[Pavel Machek]

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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