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SubjectRe: test13-pre5
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 <>
> >
> > 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.


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