Re: [PATCH 2/5] writeback: dirty position control

[Wu Fengguang <fengguang.wu@intel.com>]

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
 |
 |
 |   *
 |      *
 |         *
 |            *
 |               *
 |                  *
 |                     *
 |                        *
 |                           *
 |                              *
 |                                 *
 .. pos_ratio = 1.0 ..................*
 |                                    .  *
 |                                    .     *
 |                                    .        *
 |                                    .           *
 |                                    .              *
 |                                    .                 *
 |                                    .                    *
 |                                    .                       *
 |                                    .                          *
 |                                    .                             *
 |                                    .                                *
 |                                    .                                   *
 |                                    .                                      *
 |                                    .                                         *
 |                                    .                                            *
 |                                    .                                               *
 +------------------------------------.--------------------------------------------------*---------------------->
 0                                   goal                                              origin         dirty pages

Thanks,
Fengguang

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