* [PATCH 11/13] writeback: scale down max throttle bandwidth on concurrent dirtiers
@ 2010-11-17 3:58 Wu Fengguang
0 siblings, 0 replies; 2+ messages in thread
From: Wu Fengguang @ 2010-11-17 3:58 UTC (permalink / raw)
To: Andrew Morton
Cc: Theodore Ts'o, Wu Fengguang, Dave Chinner, Jan Kara,
Peter Zijlstra, Mel Gorman, Rik van Riel, KOSAKI Motohiro,
Chris Mason, Christoph Hellwig, linux-mm, linux-fsdevel, LKML
Andrew,
References: <20101117035821.000579293@intel.com>
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.
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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^ permalink raw reply [flat|nested] 2+ messages in thread* [PATCH 00/13] IO-less dirty throttling v2
@ 2010-11-17 4:27 Wu Fengguang
2010-11-17 4:27 ` [PATCH 11/13] writeback: scale down max throttle bandwidth on concurrent dirtiers Wu Fengguang
0 siblings, 1 reply; 2+ messages in thread
From: Wu Fengguang @ 2010-11-17 4:27 UTC (permalink / raw)
To: Andrew Morton
Cc: Jan Kara, Christoph Hellwig, Dave Chinner, Theodore Ts'o,
Chris Mason, Peter Zijlstra, Mel Gorman, Rik van Riel,
KOSAKI Motohiro, Wu Fengguang, linux-mm, linux-fsdevel, LKML
Andrew,
This is a revised subset of "[RFC] soft and dynamic dirty throttling limits"
<http://thread.gmane.org/gmane.linux.kernel.mm/52966>.
The basic idea is to introduce a small region under the bdi dirty threshold.
The task will be throttled gently when stepping into the bottom of region,
and get throttled more and more aggressively as bdi dirty+writeback pages
goes up closer to the top of region. At some point the application will be
throttled at the right bandwidth that balances with the device write bandwidth.
(the first patch and documentation has more details)
Changes from initial RFC:
- adaptive rate limiting, to reduce overheads when under throttle threshold
- prevent overrunning dirty limit on lots of concurrent dirtiers
- add Documentation/filesystems/writeback-throttling-design.txt
- lower max pause time from 200ms to 100ms; min pause time from 10ms to 1jiffy
- don't drop the laptop mode code
- update and comment the trace event
- benchmarks on concurrent dd and fs_mark covering both large and tiny files
- bdi->write_bandwidth updates should be rate limited on concurrent dirtiers,
otherwise it will drift fast and fluctuate
- don't call balance_dirty_pages_ratelimit() when writing to already dirtied
pages, otherwise the task will be throttled too much
The patches are based on 2.6.37-rc2 and Jan's sync livelock patches. For easier
access I put them in
git://git.kernel.org/pub/scm/linux/kernel/git/wfg/writeback.git dirty-throttling-v2
Wu Fengguang (12):
writeback: IO-less balance_dirty_pages()
writeback: consolidate variable names in balance_dirty_pages()
writeback: per-task rate limit on balance_dirty_pages()
writeback: prevent duplicate balance_dirty_pages_ratelimited() calls
writeback: bdi write bandwidth estimation
writeback: show bdi write bandwidth in debugfs
writeback: quit throttling when bdi dirty pages dropped
writeback: reduce per-bdi dirty threshold ramp up time
writeback: make reasonable gap between the dirty/background thresholds
writeback: scale down max throttle bandwidth on concurrent dirtiers
writeback: add trace event for balance_dirty_pages()
writeback: make nr_to_write a per-file limit
Jan Kara (1):
writeback: account per-bdi accumulated written pages
.../filesystems/writeback-throttling-design.txt | 210 +++++++++++++
fs/fs-writeback.c | 16 +
include/linux/backing-dev.h | 3 +
include/linux/sched.h | 7 +
include/linux/writeback.h | 14 +
include/trace/events/writeback.h | 61 ++++-
mm/backing-dev.c | 29 +-
mm/filemap.c | 5 +-
mm/memory_hotplug.c | 3 -
mm/page-writeback.c | 320 +++++++++++---------
10 files changed, 511 insertions(+), 157 deletions(-)
It runs smoothly on typical configurations. Under small memory system the pause
time will fluctuate much more due to the limited range for soft throttling.
The soft dirty threshold is now lowered to (background + dirty)/2=15%. So it
will be throttling the applications a bit earlier, and may be perceived by end
users as performance "slow down" if his application happens to dirty a bit more
than 15%. Note that vanilla kernel also has this limit at fresh boot: it starts
checking bdi limits when exceeding the global 15%, however the bdi limit ramps
up pretty slowly in common configurations, so the task is immediately throttled.
The task's think time is not considered for now when computing the pause time.
So it will throttle an "scp" over network way harder than a local "cp". When
to take the user space think time into account and ensure accurate throttle
bandwidth, we will effectively create a simple write I/O bandwidth controller.
On a simple test of 100 dd, it reduces the CPU %system time from 30% to 3%, and
improves IO throughput from 38MB/s to 42MB/s.
The fs_mark benchmark is interesting. The CPU overheads are almost reduced by
half. Before patch the benchmark is actually bounded by CPU. After patch it's
IO bound, but strangely the throughput becomes slightly slower.
# ./fs_mark -D 10000 -S0 -n 100000 -s 1 -L 63 -d /mnt/scratch/0 -d /mnt/scratch/1 -d /mnt/scratch/2 -d /mnt/scratch/3 -d /mnt/scratch/4 -d /mnt/scratch/5 -d /mnt/scratch/6 -d /mnt/scratch/7 -d /mnt/scratch/8 -d /mnt/scratch/9 -d /mnt/scratch/10 -d /mnt/scratch/11
# Version 3.3, 12 thread(s) starting at Thu Nov 11 21:01:36 2010
# Sync method: NO SYNC: Test does not issue sync() or fsync() calls.
# Directories: Time based hash between directories across 10000 subdirectories with 180 seconds per subdirectory.
# File names: 40 bytes long, (16 initial bytes of time stamp with 24 random bytes at end of name)
# Files info: size 1 bytes, written with an IO size of 16384 bytes per write
# App overhead is time in microseconds spent in the test not doing file writing related system calls.
#
2.6.36
FSUse% Count Size Files/sec App Overhead
0 1200000 1 1261.7 524762513
0 2400000 1 1195.3 537844546
0 3600000 1 1231.9 496441566
1 4800000 1 1175.8 552421522
1 6000000 1 1191.6 558529735
1 7200000 1 1165.3 551178395
2 8400000 1 1175.0 533209632
2 9600000 1 1200.6 534862246
2 10800000 1 1181.2 540616486
2 12000000 1 1137.4 554551797
3 13200000 1 1143.7 563319651
3 14400000 1 1169.0 519527533
3 15600000 1 1184.0 533550370
4 16800000 1 1161.3 534358727
4 18000000 1 1193.4 521610050
4 19200000 1 1177.6 524117437
5 20400000 1 1172.6 506166634
5 21600000 1 1172.3 515725633
avg 1182.761 533488581.833
2.6.36+
FSUse% Count Size Files/sec App Overhead
0 1200000 1 1125.0 357885976
0 2400000 1 1155.6 288103795
0 3600000 1 1172.4 296521755
1 4800000 1 1136.0 301718887
1 6000000 1 1156.7 303605077
1 7200000 1 1102.9 288852150
2 8400000 1 1140.9 294894485
2 9600000 1 1148.0 314394450
2 10800000 1 1099.7 296365560
2 12000000 1 1153.6 316283083
3 13200000 1 1087.9 339988006
3 14400000 1 1183.9 270836344
3 15600000 1 1122.7 276400918
4 16800000 1 1132.1 285272223
4 18000000 1 1154.8 283424055
4 19200000 1 1202.5 294558877
5 20400000 1 1158.1 293971332
5 21600000 1 1159.4 287720335
5 22800000 1 1150.1 282987509
5 24000000 1 1150.7 283870613
6 25200000 1 1123.8 288094185
6 26400000 1 1152.1 296984323
6 27600000 1 1190.7 282403174
7 28800000 1 1088.6 290493643
7 30000000 1 1144.1 290311419
7 31200000 1 1186.0 290021271
7 32400000 1 1213.9 279465138
8 33600000 1 1117.3 275745401
avg 1146.768 294684785.143
I noticed that
1) BdiWriteback can grow very large. For example, bdi 8:16 has 72960KB
writeback pages, however the disk IO queue can hold at most
nr_request*max_sectors_kb=128*512kb=64MB writeback pages. Maybe xfs manages
to create perfect sequential layouts and writes, and the other 8MB writeback
pages are flying inside the disk?
root@wfg-ne02 /cc/fs_mark-3.3/ne02-2.6.36+# g BdiWriteback /debug/bdi/8:*/*
/debug/bdi/8:0/stats:BdiWriteback: 0 kB
/debug/bdi/8:112/stats:BdiWriteback: 68352 kB
/debug/bdi/8:128/stats:BdiWriteback: 62336 kB
/debug/bdi/8:144/stats:BdiWriteback: 61824 kB
/debug/bdi/8:160/stats:BdiWriteback: 67328 kB
/debug/bdi/8:16/stats:BdiWriteback: 72960 kB
/debug/bdi/8:176/stats:BdiWriteback: 57984 kB
/debug/bdi/8:192/stats:BdiWriteback: 71936 kB
/debug/bdi/8:32/stats:BdiWriteback: 68352 kB
/debug/bdi/8:48/stats:BdiWriteback: 56704 kB
/debug/bdi/8:64/stats:BdiWriteback: 50304 kB
/debug/bdi/8:80/stats:BdiWriteback: 68864 kB
/debug/bdi/8:96/stats:BdiWriteback: 2816 kB
2) the 12 disks are not all 100% utilized. Not even close: sdd, sdf, sdh, sdj
are almost idle at the moment. Dozens of seconds later, some other disks
become idle. This happens both before/after patch. There may be some hidden
bugs (unrelated to this patchset).
avg-cpu: %user %nice %system %iowait %steal %idle
0.17 0.00 97.87 1.08 0.00 0.88
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdc 0.00 63.00 0.00 125.00 0.00 1909.33 30.55 3.88 31.65 6.57 82.13
sdd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sde 0.00 19.00 0.00 112.00 0.00 1517.17 27.09 3.95 35.33 8.00 89.60
sdg 0.00 92.67 0.33 126.00 2.67 1773.33 28.12 14.83 120.78 7.73 97.60
sdf 0.00 32.33 0.00 91.67 0.00 1408.17 30.72 4.84 52.97 7.72 70.80
sdh 0.00 17.67 0.00 5.00 0.00 124.00 49.60 0.07 13.33 9.60 4.80
sdi 0.00 44.67 0.00 5.00 0.00 253.33 101.33 0.15 29.33 10.93 5.47
sdl 0.00 168.00 0.00 135.67 0.00 2216.33 32.67 6.41 45.42 5.75 78.00
sdk 0.00 225.00 0.00 123.00 0.00 2355.83 38.31 9.50 73.03 6.94 85.33
sdj 0.00 1.00 0.00 2.33 0.00 26.67 22.86 0.01 2.29 1.71 0.40
sdb 0.00 14.33 0.00 101.67 0.00 1278.00 25.14 2.02 19.95 7.16 72.80
sdm 0.00 150.33 0.00 144.33 0.00 2344.50 32.49 5.43 33.94 5.39 77.73
avg-cpu: %user %nice %system %iowait %steal %idle
0.12 0.00 98.63 0.83 0.00 0.42
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdc 0.00 105.67 0.00 127.33 0.00 1810.17 28.43 4.39 32.43 6.67 84.93
sdd 0.00 5.33 0.00 10.67 0.00 128.00 24.00 0.03 2.50 1.25 1.33
sde 0.00 180.33 0.33 107.67 2.67 2109.33 39.11 8.11 73.93 8.99 97.07
sdg 0.00 7.67 0.00 63.67 0.00 1387.50 43.59 1.45 24.29 11.08 70.53
sdf 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdh 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdi 0.00 62.67 0.00 94.67 0.00 1743.50 36.83 3.28 34.68 8.52 80.67
sdl 0.00 162.00 0.00 141.67 0.00 2295.83 32.41 7.09 51.79 6.14 86.93
sdk 0.00 34.33 0.00 143.67 0.00 1910.17 26.59 5.07 38.90 6.26 90.00
sdj 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdb 0.00 195.00 0.00 96.67 0.00 1949.50 40.33 5.54 57.23 8.39 81.07
sdm 0.00 155.00 0.00 143.00 0.00 2357.50 32.97 5.21 39.98 5.71 81.60
Thanks,
Fengguang
--
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^ permalink raw reply [flat|nested] 2+ messages in thread* [PATCH 11/13] writeback: scale down max throttle bandwidth on concurrent dirtiers
2010-11-17 4:27 [PATCH 00/13] IO-less dirty throttling v2 Wu Fengguang
@ 2010-11-17 4:27 ` Wu Fengguang
0 siblings, 0 replies; 2+ messages in thread
From: Wu Fengguang @ 2010-11-17 4:27 UTC (permalink / raw)
To: Andrew Morton
Cc: Jan Kara, Wu Fengguang, Christoph Hellwig, Dave Chinner,
Theodore Ts'o, Chris Mason, Peter Zijlstra, Mel Gorman,
Rik van Riel, KOSAKI Motohiro, linux-mm, linux-fsdevel, LKML
[-- Attachment #1: writeback-adaptive-throttle-bandwidth.patch --]
[-- Type: text/plain, Size: 4042 bytes --]
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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