From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 90C4A6B0093 for ; Tue, 14 Dec 2010 02:00:11 -0500 (EST) Date: Tue, 14 Dec 2010 15:00:05 +0800 From: Wu Fengguang Subject: Re: [PATCH 12/35] writeback: scale down max throttle bandwidth on concurrent dirtiers Message-ID: <20101214070005.GB6940@localhost> References: <20101213144646.341970461@intel.com> <20101213150327.809762057@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org To: "Yan, Zheng" Cc: Andrew Morton , Jan Kara , Christoph Hellwig , Trond Myklebust , Dave Chinner , Theodore Ts'o , Chris Mason , Peter Zijlstra , Mel Gorman , Rik van Riel , KOSAKI Motohiro , Greg Thelen , Minchan Kim , linux-mm , "linux-fsdevel@vger.kernel.org" , LKML List-ID: On Tue, Dec 14, 2010 at 09:21:19AM +0800, Yan Zheng wrote: > On Mon, Dec 13, 2010 at 10:46 PM, Wu Fengguang wrote: > > This will noticeably reduce the fluctuaions of pause time when there are > > 100+ concurrent dirtiers. > > > > The more parallel dirtiers (1 dirtier => 4 dirtiers), the smaller > > bandwidth each dirtier will share (bdi_bandwidth => bdi_bandwidth/4), > > the less gap to the dirty limit ((C-A) => (C-B)), the less stable the > > pause time will be (given the same fluctuation of bdi_dirty). > > > > For example, if A drifts to A', its pause time may drift from 5ms to > > 6ms, while B to B' may drift from 50ms to 90ms. A It's much larger > > fluctuations in relative ratio as well as absolute time. > > > > Fig.1 before patch, gap (C-B) is too low to get smooth pause time > > > > throttle_bandwidth_A = bdi_bandwidth .........o > > A A A A A A A A A A A A A A A A A A A A A A A | o <= A' > > A A A A A A A A A A A A A A A A A A A A A A A | A o > > A A A A A A A A A A A A A A A A A A A A A A A | A A o > > A A A A A A A A A A A A A A A A A A A A A A A | A A A o > > A A A A A A A A A A A A A A A A A A A A A A A | A A A A o > > throttle_bandwidth_B = bdi_bandwidth / 4 .....|...........o > > A A A A A A A A A A A A A A A A A A A A A A A | A A A A A | o <= B' > > ----------------------------------------------+-----------+---o > > A A A A A A A A A A A A A A A A A A A A A A A A A A A A A B A C > > > > The solution is to lower the slope of the throttle line accordingly, > > which makes B stabilize at some point more far away from C. > > > > Fig.2 after patch > > > > throttle_bandwidth_A = bdi_bandwidth .........o > > A A A A A A A A A A A A A A A A A A A A A A A | o <= A' > > A A A A A A A A A A A A A A A A A A A A A A A | A o > > A A A A A A A A A A A A A A A A A A A A A A A | A A o > > A A lowered max throttle bandwidth for B ===> * A A A o > > A A A A A A A A A A A A A A A A A A A A A A A | A * A A o > > throttle_bandwidth_B = bdi_bandwidth / 4 .............* A o > > A A A A A A A A A A A A A A A A A A A A A A A | A A A | A * o > > ----------------------------------------------+-------+-------o > > A A A A A A A A A A A A A A A A A A A A A A A A A A A B A A A C > > > > Note that C is actually different points for 1-dirty and 4-dirtiers > > cases, but for easy graphing, we move them together. > > > > Signed-off-by: Wu Fengguang > > --- > > A mm/page-writeback.c | A 16 +++++++++++++--- > > A 1 file changed, 13 insertions(+), 3 deletions(-) > > > > --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:14.000000000 +0800 > > +++ linux-next/mm/page-writeback.c A A A 2010-12-13 21:46:15.000000000 +0800 > > @@ -587,6 +587,7 @@ static void balance_dirty_pages(struct a > > A A A A unsigned long background_thresh; > > A A A A unsigned long dirty_thresh; > > A A A A unsigned long bdi_thresh; > > + A A A unsigned long task_thresh; > > A A A A unsigned long long bw; > > A A A A unsigned long period; > > A A A A unsigned long pause = 0; > > @@ -616,7 +617,7 @@ static void balance_dirty_pages(struct a > > A A A A A A A A A A A A break; > > > > A A A A A A A A bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh, nr_dirty); > > - A A A A A A A bdi_thresh = task_dirty_limit(current, bdi_thresh); > > + A A A A A A A task_thresh = task_dirty_limit(current, bdi_thresh); > > > > A A A A A A A A /* > > A A A A A A A A * In order to avoid the stacked BDI deadlock we need > > @@ -638,14 +639,23 @@ static void balance_dirty_pages(struct a > > > > A A A A A A A A bdi_update_bandwidth(bdi, start_time, bdi_dirty, bdi_thresh); > > > > - A A A A A A A if (bdi_dirty >= bdi_thresh || nr_dirty > dirty_thresh) { > > + A A A A A A A if (bdi_dirty >= task_thresh || nr_dirty > dirty_thresh) { > > A A A A A A A A A A A A pause = MAX_PAUSE; > > A A A A A A A A A A A A goto pause; > > A A A A A A A A } > > > > + A A A A A A A /* > > + A A A A A A A A * When bdi_dirty grows closer to bdi_thresh, it indicates more > > + A A A A A A A A * concurrent dirtiers. Proportionally lower the max throttle > > + A A A A A A A A * bandwidth. This will resist bdi_dirty from approaching to > > + A A A A A A A A * close to task_thresh, and help reduce fluctuations of pause > > + A A A A A A A A * time when there are lots of dirtiers. > > + A A A A A A A A */ > > A A A A A A A A bw = bdi->write_bandwidth; > > - > > A A A A A A A A bw = bw * (bdi_thresh - bdi_dirty); > > + A A A A A A A do_div(bw, bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1); > > + > > + A A A A A A A bw = bw * (task_thresh - bdi_dirty); > > A A A A A A A A do_div(bw, bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1); > > Maybe changing this line to "do_div(bw, task_thresh / > TASK_SOFT_DIRTY_LIMIT + 1);" > is more consistent. I'll show you another consistency of "shape" :) http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/slides/light-dirtier-control-line.svg http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/slides/heavy-dirtier-control-line.svg In the above two figures, the overall control lines for light/heavy dirtier tasks have exactly the same shape -- it's merely shifted in the X axis direction. So the current form is actually more simple. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org