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Date: Tue, 14 Dec 2010 09:21:19 +0800 From: "Yan, Zheng" <zheng.z.yan@...ux.intel.com> To: Wu Fengguang <fengguang.wu@...el.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, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 12/35] writeback: scale down max throttle bandwidth on concurrent dirtiers 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. Thanks Yan, Zheng -- 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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