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

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Message-ID: <20070510214014.GA4163@kernel.dk>
Date:	Thu, 10 May 2007 23:40:15 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alasdair G Kergon <agk@...hat.com>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org, Heinz Mauelshagen <hjm@...hat.com>
Subject: Re: [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@...ux01.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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