[dm-devel] Re: [2.6.22 PATCH 22/26] dm: bio list helpers

[Alan Cox]

On Thu, 10 May 2007 16:17:57 +0200 (MEST)
Jan Engelhardt <jengelh at 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...

Alan