[PATCH 12/35] writeback: scale down max throttle bandwidth on concurrent dirtiers

[Wu Fengguang]

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@xxxxxxxxx>
---
 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);
 
 		period = HZ * pages_dirtied / ((unsigned long)bw + 1) + 1;


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