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

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Message-ID: <20070510152957.5c1ddec9@the-village.bc.nu>
Date:	Thu, 10 May 2007 15:29:57 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	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, 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...

Alan
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