Messages in this thread |  | | Date | Thu, 28 Dec 2000 21:25:36 -0200 (BRDT) | From | Rik van Riel <> | 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 -- Hollywood goes for world dumbination, Trailer at 11.
http://www.surriel.com/ http://www.conectiva.com/ http://distro.conectiva.com.br/
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