The patch titled
writeback: scale down max throttle bandwidth on concurrent dirtiers
has been removed from the -mm tree. Its filename was
writeback-scale-down-max-throttle-bandwidth-on-concurrent-dirtiers.patch
This patch was dropped because an updated version will be merged
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: writeback: scale down max throttle bandwidth on concurrent dirtiers
From: Wu Fengguang <fengguang.wu@xxxxxxxxx>
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>
Cc: Chris Mason <chris.mason@xxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Li Shaohua <shaohua.li@xxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: Richard Kennedy <richard@xxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Mel Gorman <mel@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Michael Rubin <mrubin@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page-writeback.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff -puN mm/page-writeback.c~writeback-scale-down-max-throttle-bandwidth-on-concurrent-dirtiers mm/page-writeback.c
--- a/mm/page-writeback.c~writeback-scale-down-max-throttle-bandwidth-on-concurrent-dirtiers
+++ a/mm/page-writeback.c
@@ -538,6 +538,7 @@ 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;
@@ -567,7 +568,7 @@ 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
@@ -606,14 +607,23 @@ 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);
_
Patches currently in -mm which might be from fengguang.wu@xxxxxxxxx are
origin.patch
writeback-add-trace-event-for-balance_dirty_pages.patch
writeback-make-nr_to_write-a-per-file-limit.patch
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