From mboxrd@z Thu Jan  1 00:00:00 1970
From: Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [PATCH 04/35] writeback: reduce per-bdi dirty threshold ramp
 up time
Date: Tue, 14 Dec 2010 22:39:02 +0800
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Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Christoph Hellwig <hch@lst.de>,
	Trond Myklebust <Trond.Myklebust@netapp.com>,
	Dave Chinner <david@fromorbit.com>,
	Theodore Ts'o <tytso@mit.edu>,
	Chris Mason <chris.mason@oracle.com>,
	Mel Gorman <mel@csn.ul.ie>, Rik van Riel <riel@redhat.com>,
	KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Greg Thelen <gthelen@google.com>,
	Minchan Kim <minchan.kim@gmail.com>,
	linux-mm <linux-mm@kvack.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
To: Richard Kennedy <richard@rsk.demon.co.uk>
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On Tue, Dec 14, 2010 at 10:33:25PM +0800, Wu Fengguang wrote:
> On Tue, Dec 14, 2010 at 09:59:10PM +0800, Wu Fengguang wrote:
> > On Tue, Dec 14, 2010 at 09:37:34PM +0800, Richard Kennedy wrote:
> 
> > > As to the ramp up time, when writing to 2 disks at the same time I see
> > > the per_bdi_threshold taking up to 20 seconds to converge on a steady
> > > value after one of the write stops. So I think this could be speeded up
> > > even more, at least on my setup.
> > 
> > I have the roughly same ramp up time on the 1-disk 3GB mem test:
> > 
> > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/dirty-pages.png
> >  
> 
> Interestingly, the above graph shows that after about 10s fast ramp
> up, there is another 20s slow ramp down. It's obviously due the
> decline of global limit:
> 
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/vmstat-dirty.png
> 
> But why is the global limit declining?  The following log shows that
> nr_file_pages keeps growing and goes stable after 75 seconds (so long
> time!). In the same period nr_free_pages goes slowly down to its
> stable value. Given that the global limit is mainly derived from
> nr_free_pages+nr_file_pages (I disabled swap), something must be
> slowly eating memory until 75 ms. Maybe the tracing ring buffers?
> 
>          free     file      reclaimable pages
> 50s      369324 + 318760 => 688084
> 60s      235989 + 448096 => 684085
> 
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/vmstat

The log shows that ~64MB reclaimable memory is stoled. But the trace
data only takes 1.8MB. Hmm..

Thanks,
Fengguang