linux-kernel - Re: [PATCH 04/35] writeback: reduce per-bdi dirty threshold ramp up time

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Message-ID: <20101214143325.GA22764@localhost>
Date:	Tue, 14 Dec 2010 22:33:25 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	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>, Rik van Riel <riel@...hat.com>,
	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 04/35] writeback: reduce per-bdi dirty threshold ramp
 up time

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

Thanks,
Fengguang
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