From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754874Ab0LNHAO (ORCPT ); Tue, 14 Dec 2010 02:00:14 -0500 Received: from mga03.intel.com ([143.182.124.21]:38165 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751458Ab0LNHAL (ORCPT ); Tue, 14 Dec 2010 02:00:11 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.59,340,1288594800"; d="scan'208";a="361333067" Date: Tue, 14 Dec 2010 15:00:05 +0800 From: Wu Fengguang 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 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: User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.  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 > > --- > >  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