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    SubjectRe: [2.6.22 PATCH 22/26] dm: bio list helpers
    On Thu, May 10 2007, Alan Cox wrote:
    > On Thu, 10 May 2007 16:17:57 +0200 (MEST)
    > Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:
    >
    > >
    > > On May 9 2007 08:49, Jens Axboe wrote:
    > > >On Tue, May 08 2007, Andrew Morton wrote:
    > > >> > +#define bio_list_for_each(bio, bl) \
    > > >> > + for (bio = (bl)->head; bio && ({ prefetch(bio->bi_next); 1; }); \
    > > >> > + bio = bio->bi_next)
    > > >> > +
    > > >
    > > >Besides, manual prefetching is very rarely a win. I dabbled with some
    > > >benchmarks a few weeks back (with the doubly linked lists), and in most
    > > >cases it was actually a loss. So I'd vote for just removing the
    > > >prefetch() above.
    > >
    > > So is the prefetching in the basic ADTs (e.g. linux/list.h) a loss too?
    >
    > Depends on the box it seems. On the newest systems the processor
    > prefetching seems to be very much smarter. On a "classic" AMD Athlon the
    > prefetching made the scheduler about 1.5% faster...

    It very much depends on the box, indeed. The ones I tested on were
    _slower_ with the prefetching, perhaps the dumber CPU's will benefit. In
    the long run, I don't think the manual prefetching is a good idea.

    --
    Jens Axboe

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