linux-kernel - Re: [PATCH 01/35] writeback: enabling gate limit for light dirtied bdi

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Date:	Thu, 13 Jan 2011 11:58:31 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/35] writeback: enabling gate limit for light dirtied
 bdi

> sigh.. I've been hassled a lot by the possible disharmonies between
> the bdi/global dirty limits.
> 
> One example is the below graph, where the bdi dirty pages are
> constantly exceeding the bdi dirty limit. The root cause is,
> "(dirty + background) / 2" may be close to or even exceed
> bdi_dirty_limit. 

When exceeded, the task will not get throttled at all at some time,
and get hard throttled at other times.

> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/256M/ext3-2dd-1M-8p-191M-2.6.37-rc5+-2010-12-09-13-42/dirty-pages-200.png

This graph is more obvious. However I'm no longer sure they are the
exact graphs that are caused by "(dirty + background) / 2 > bdi_dirty_limit",
which evaluates to TRUE after I do "[PATCH 02/35] writeback: safety
margin for bdi stat error", which lowered bdi_dirty_limit by 1-2MB in
that test case.

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/256M/btrfs-1dd-1M-8p-191M-2.6.37-rc5+-2010-12-09-14-35/dirty-pages-200.png

Thanks,
Fengguang
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