linux-kernel - Re: [PATCH 02/10] writeback: balance_dirty_pages() shall write more than dirtied pages

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Message-Id: <1253804614.18939.35.camel@laptop>
Date:	Thu, 24 Sep 2009 17:03:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, fengguang.wu@...el.com,
	shaohua.li@...el.com, chris.mason@...cle.com, jack@...e.cz
Subject: Re: [PATCH 02/10] writeback: balance_dirty_pages() shall write
 more than dirtied pages

On Thu, 2009-09-24 at 16:40 +0200, Jens Axboe wrote:
> From: Wu Fengguang <fengguang.wu@...el.com>
> 
> Some filesystem may choose to write much more than ratelimit_pages
> before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
> determine number to write based on real number of dirtied pages.
> 
> Otherwise it is possible that
>   loop {
>     btrfs_file_write():     dirty 1024 pages
>     balance_dirty_pages():  write up to 48 pages (= ratelimit_pages * 1.5)
>   }
> in which the writeback rate cannot keep up with dirty rate, and the
> dirty pages go all the way beyond dirty_thresh.
> 
> The increased write_chunk may make the dirtier more bumpy.
> So filesystems shall be take care not to dirty too much at
> a time (eg. > 4MB) without checking the ratelimit.
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> Signed-off-by: Jens Axboe <jens.axboe@...cle.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

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