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

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Message-ID: <Pine.LNX.4.61.0705101617170.32349@yvahk01.tjqt.qr>
Date:	Thu, 10 May 2007 16:17:57 +0200 (MEST)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	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 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?



	Jan
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