Messages in this thread | | | Date | Thu, 10 May 2007 23:40:15 +0200 | From | Jens Axboe <> | Subject | Re: [2.6.22 PATCH 22/26] dm: bio list helpers |
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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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