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

Wu Fengguang
2011-08-08T23:05:49Z

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