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Date: Tue, 14 Dec 2010 15:00:05 +0800 From: Wu Fengguang <fengguang.wu@...el.com> To: "Yan, Zheng" <zheng.z.yan@...ux.intel.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>, 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>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, 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 12/35] writeback: scale down max throttle bandwidth on concurrent dirtiers On Tue, Dec 14, 2010 at 09:21:19AM +0800, Yan Zheng wrote: > On Mon, Dec 13, 2010 at 10:46 PM, Wu Fengguang <fengguang.wu@...el.com> 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. 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 > > | o <= A' > > | o > > | o > > | o > > | o > > throttle_bandwidth_B = bdi_bandwidth / 4 .....|...........o > > | | o <= B' > > ----------------------------------------------+-----------+---o > > A B 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 > > | o <= A' > > | o > > | o > > lowered max throttle bandwidth for B ===> * o > > | * o > > throttle_bandwidth_B = bdi_bandwidth / 4 .............* o > > | | * o > > ----------------------------------------------+-------+-------o > > A B 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 <fengguang.wu@...el.com> > > --- > > mm/page-writeback.c | 16 +++++++++++++--- > > 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 2010-12-13 21:46:15.000000000 +0800 > > @@ -587,6 +587,7 @@ static void balance_dirty_pages(struct a > > unsigned long background_thresh; > > unsigned long dirty_thresh; > > unsigned long bdi_thresh; > > + unsigned long task_thresh; > > unsigned long long bw; > > unsigned long period; > > unsigned long pause = 0; > > @@ -616,7 +617,7 @@ static void balance_dirty_pages(struct a > > break; > > > > bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh, nr_dirty); > > - bdi_thresh = task_dirty_limit(current, bdi_thresh); > > + task_thresh = task_dirty_limit(current, bdi_thresh); > > > > /* > > * In order to avoid the stacked BDI deadlock we need > > @@ -638,14 +639,23 @@ static void balance_dirty_pages(struct a > > > > bdi_update_bandwidth(bdi, start_time, bdi_dirty, bdi_thresh); > > > > - if (bdi_dirty >= bdi_thresh || nr_dirty > dirty_thresh) { > > + if (bdi_dirty >= task_thresh || nr_dirty > dirty_thresh) { > > pause = MAX_PAUSE; > > goto pause; > > } > > > > + /* > > + * When bdi_dirty grows closer to bdi_thresh, it indicates more > > + * concurrent dirtiers. Proportionally lower the max throttle > > + * bandwidth. This will resist bdi_dirty from approaching to > > + * close to task_thresh, and help reduce fluctuations of pause > > + * time when there are lots of dirtiers. > > + */ > > bw = bdi->write_bandwidth; > > - > > bw = bw * (bdi_thresh - bdi_dirty); > > + do_div(bw, bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1); > > + > > + bw = bw * (task_thresh - bdi_dirty); > > 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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