writeback: scale down max throttle bandwidth on concurrent dirtiers

Andrew,
References: <[email protected]>
Content-Disposition: inline; filename=writeback-adaptive-throttle-bandwidth.patch

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.

---
 mm/page-writeback.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

@@ -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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