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Message-ID: <20101214130018.GA19424@localhost>
Date: Tue, 14 Dec 2010 21:00:18 +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 03:00:05PM +0800, Wu Fengguang wrote:
> 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.
Sorry it's not the overall control lines that's simply shifted, but
the task control line.
bdi control line:
> > > bw = bw * (bdi_thresh - bdi_dirty);
> > > + do_div(bw, bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1);
task control line:
> > > + bw = bw * (task_thresh - bdi_dirty);
> > > do_div(bw, bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1);
The use of bdi_thresh in the last line makes sure all task control
lines are of the same slope.
Thanks,
Fengguang
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