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Subject[PATCH 11/13] writeback: scale down max throttle bandwidth on concurrent dirtiers
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@intel.com>
---
mm/page-writeback.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
--- linux-next.orig/mm/page-writeback.c 2010-11-15 19:52:43.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-11-15 21:30:45.000000000 +0800
@@ -537,6 +537,7 @@ 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 bw;
unsigned long pause = 0;
bool dirty_exceeded = false;
@@ -566,7 +567,7 @@ static void balance_dirty_pages(struct a
break;

bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
- 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
@@ -605,14 +606,23 @@ static void balance_dirty_pages(struct a
break;
bdi_prev_dirty = bdi_dirty;

- if (bdi_dirty >= bdi_thresh) {
+ if (bdi_dirty >= task_thresh) {
pause = HZ/10;
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);
+ bw = bw / (bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1);
+
+ bw = bw * (task_thresh - bdi_dirty);
bw = bw / (bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1);

pause = HZ * (pages_dirtied << PAGE_CACHE_SHIFT) / (bw + 1);



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