linux-kernel - Re: [patch] x86, mm: pass in 'total' to __copy_from_user

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Date:	Sun, 1 Mar 2009 03:07:08 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>,
	torvalds@...ux-foundation.org, mingo@...e.hu,
	nickpiggin@...oo.com.au, sqazi@...gle.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

On Sat, Feb 28, 2009 at 05:38:13PM -0800, Arjan van de Ven wrote:
> On Sun, 1 Mar 2009 02:48:22 +0100
> Andi Kleen <andi@...stfloor.org> wrote:
> 
> > > the entire point of using movntq and friends was to save half the
> > 
> > I thought the point was to not pollute caches? At least that is 
> > what I remember being told when I merged the patch.
> > 
> 
> the reason that movntq and co are faster is because you avoid the
> write-allocate behavior of the caches....

Not faster than rep ; movs which does similar magic anyways. Of course it being
magic it can vary a lot and is somewhat unpredictable.

Also in my experince movnt is not actually that much faster for small 
transfers (as in what the kernel mostly does) 

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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