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Date: Tue, 9 Aug 2011 07:05:35 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
Dave Chinner <david@...morbit.com>,
Greg Thelen <gthelen@...gle.com>,
Minchan Kim <minchan.kim@...il.com>,
Vivek Goyal <vgoyal@...hat.com>,
Andrea Righi <arighi@...eler.com>,
linux-mm <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] writeback: dirty position control
On Mon, Aug 08, 2011 at 10:41:41PM +0800, Peter Zijlstra wrote:
> On Mon, 2011-08-08 at 22:11 +0800, Wu Fengguang wrote:
> > @@ -538,11 +538,6 @@ static unsigned long bdi_position_ratio(
> > goal = thresh - thresh / DIRTY_SCOPE;
> > origin = 4 * thresh;
> >
> > - if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
> > - origin = limit; /* auxiliary control line */
> > - goal = (goal + origin) / 2;
> > - pos_ratio >>= 1;
> > - }
> > pos_ratio = origin - dirty;
> > pos_ratio <<= BANDWIDTH_CALC_SHIFT;
> > do_div(pos_ratio, origin - goal + 1);
FYI I've updated the fix to the below one, so that @limit will be used
as the origin in the rare case of (4*thresh < dirty).
--- linux-next.orig/mm/page-writeback.c 2011-08-08 21:56:11.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-08-09 06:34:25.000000000 +0800
@@ -536,13 +536,8 @@ static unsigned long bdi_position_ratio(
* global setpoint
*/
goal = thresh - thresh / DIRTY_SCOPE;
- origin = 4 * thresh;
+ origin = max(4 * thresh, limit);
- if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
- origin = limit; /* auxiliary control line */
- goal = (goal + origin) / 2;
- pos_ratio >>= 1;
- }
pos_ratio = origin - dirty;
pos_ratio <<= BANDWIDTH_CALC_SHIFT;
do_div(pos_ratio, origin - goal + 1);
> So basically, pos_ratio = (4t - d) / (25/8)t, which if I'm not mistaken
> comes out at 32/25 - 8d/25t. Which simply doesn't make sense at all.
This is the more meaningful view :)
origin - dirty
pos_ratio = --------------
origin - goal
which comes from the below [*] control line, so that when (dirty == goal),
pos_ratio == 1.0:
^ pos_ratio
|
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| *
| *
| *
| *
| *
| *
| *
| *
| *
| *
| *
.. pos_ratio = 1.0 ..................*
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
| . *
+------------------------------------.--------------------------------------------------*---------------------->
0 goal origin dirty pages
Thanks,
Fengguang
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