lkml.org 
[lkml]   [2000]   [Dec]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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 <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/

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 12:52    [W:0.173 / U:0.172 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site