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Date: Thu, 28 Dec 2000 21:25:36 -0200 (BRDT)
From: Rik van Riel <riel@conectiva.com.br>
To: Andi Kleen <ak@suse.de>
cc: "David S. Miller" <davem@redhat.com>, torvalds@transmeta.com,
        marcelo@conectiva.com.br, linux-kernel@vger.kernel.org
Subject: Re: test13-pre5
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On Fri, 29 Dec 2000, Andi Kleen wrote:
> On Thu, Dec 28, 2000 at 02:54:52PM -0800, David S. Miller wrote:
> >    Date: Thu, 28 Dec 2000 23:58:36 +0100
> >    From: Andi Kleen <ak@suse.de>
> > 
> >    Why exactly a power of two ? To get rid of ->index ? 
> > 
> > To make things like "page - mem_map" et al. use shifts instead of
> > expensive multiplies...
> 
> I thought that is what ->index is for ? 

Nope, ->index is there to identify which offset the page
has in ->mapping, read mm/filemap.c::__find_page_nolock()
for more info.

> Also gcc seems to be already quite clever at dividing through
> small integers, e.g. using mul and shift and sub, so it may not
> be even worth to reach for a real power-of-two.
> 
> I suspect doing the arithmetics is at least faster than eating the 
> cache misses because of ->index. 

I'm pretty confident that arithmetic is faster than cache
misses ... but an unlucky size of the page struct will cause
extra cache misses due to misalignment.

regards,

Rik
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