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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 <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.

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