It would be useful for analyzing the dynamics of the throttling
algorithms, and helpful for debugging user reported problems.
Here is an interesting test to verify the theory with balance_dirty_pages()
tracing. On a partition that can do ~60MB/s, a sparse file is created and
4 rsync tasks with different write bandwidth started:
dd if=/dev/zero of=/mnt/1T bs=1M count=1 seek=1024000
echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable
rsync localhost:/mnt/1T /mnt/a --bwlimit 10000&
rsync localhost:/mnt/1T /mnt/A --bwlimit 10000&
rsync localhost:/mnt/1T /mnt/b --bwlimit 20000&
rsync localhost:/mnt/1T /mnt/c --bwlimit 30000&
Trace outputs (an old version) within 0.1 second, grouped by tasks:
rsync-3824 [004] 15002.076447: balance_dirty_pages: bdi=btrfs-2 weight=15% limit=130876 gap=5340 dirtied=192 pause=20
rsync-3822 [003] 15002.091701: balance_dirty_pages: bdi=btrfs-2 weight=15% limit=130777 gap=5113 dirtied=192 pause=20
rsync-3821 [006] 15002.004667: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129570 gap=3714 dirtied=64 pause=8
rsync-3821 [006] 15002.012654: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129589 gap=3733 dirtied=64 pause=8
rsync-3821 [006] 15002.021838: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129604 gap=3748 dirtied=64 pause=8
rsync-3821 [004] 15002.091193: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129583 gap=3983 dirtied=64 pause=8
rsync-3821 [004] 15002.102729: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129594 gap=3802 dirtied=64 pause=8
rsync-3821 [000] 15002.109252: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129619 gap=3827 dirtied=64 pause=8
rsync-3823 [002] 15002.009029: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128762 gap=2842 dirtied=64 pause=12
rsync-3823 [002] 15002.021598: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128813 gap=3021 dirtied=64 pause=12
rsync-3823 [003] 15002.032973: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128805 gap=2885 dirtied=64 pause=12
rsync-3823 [003] 15002.048800: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128823 gap=2967 dirtied=64 pause=12
rsync-3823 [003] 15002.060728: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128821 gap=3221 dirtied=64 pause=12
rsync-3823 [000] 15002.073152: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128825 gap=3225 dirtied=64 pause=12
rsync-3823 [005] 15002.090111: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128782 gap=3214 dirtied=64 pause=12
rsync-3823 [004] 15002.102520: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128764 gap=3036 dirtied=64 pause=12
The data vividly show that
- the heaviest writer is throttled a bit (weight=39%)
- the lighter writers run at full speed (weight=15%,15%,30%)
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
include/trace/events/writeback.h | 80 +++++++++++++++++++++++++++++
mm/page-writeback.c | 20 +++++++
2 files changed, 100 insertions(+)
--- linux-next.orig/include/trace/events/writeback.h 2011-04-16 11:28:27.000000000 +0800
+++ linux-next/include/trace/events/writeback.h 2011-04-16 11:28:28.000000000 +0800
@@ -197,6 +197,86 @@ TRACE_EVENT(dirty_ratelimit,
)
);
+TRACE_EVENT(balance_dirty_pages,
+
+ TP_PROTO(struct backing_dev_info *bdi,
+ unsigned long thresh,
+ unsigned long dirty,
+ unsigned long bdi_dirty,
+ unsigned long base_bw,
+ unsigned long task_bw,
+ unsigned long dirtied,
+ unsigned long period,
+ long pause,
+ unsigned long start_time),
+
+ TP_ARGS(bdi, thresh, dirty, bdi_dirty,
+ base_bw, task_bw, dirtied, period, pause, start_time),
+
+ TP_STRUCT__entry(
+ __array( char, bdi, 32)
+ __field(unsigned long, limit)
+ __field(unsigned long, goal)
+ __field(unsigned long, dirty)
+ __field(unsigned long, bdi_goal)
+ __field(unsigned long, bdi_dirty)
+ __field(unsigned long, avg_dirty)
+ __field(unsigned long, base_bw)
+ __field(unsigned long, task_bw)
+ __field(unsigned int, dirtied)
+ __field(unsigned int, dirtied_pause)
+ __field(unsigned long, period)
+ __field( long, think)
+ __field( long, pause)
+ __field(unsigned long, paused)
+ ),
+
+ TP_fast_assign(
+ strlcpy(__entry->bdi, dev_name(bdi->dev), 32);
+
+ __entry->limit = default_backing_dev_info.dirty_threshold;
+ __entry->goal = thresh - thresh / DIRTY_SCOPE;
+ __entry->dirty = dirty;
+ __entry->bdi_goal = bdi->dirty_threshold -
+ bdi->dirty_threshold / DIRTY_SCOPE;
+ __entry->bdi_dirty = bdi_dirty;
+ __entry->avg_dirty = bdi->avg_dirty;
+ __entry->base_bw = KBps(base_bw);
+ __entry->task_bw = KBps(task_bw);
+ __entry->dirtied = dirtied;
+ __entry->dirtied_pause = current->nr_dirtied_pause;
+ __entry->think = current->paused_when == 0 ? 0 :
+ (long)(jiffies - current->paused_when) * 1000 / HZ;
+ __entry->period = period * 1000 / HZ;
+ __entry->pause = pause * 1000 / HZ;
+ __entry->paused = (jiffies - start_time) * 1000 / HZ;
+ ),
+
+
+ TP_printk("bdi %s: "
+ "limit=%lu goal=%lu dirty=%lu "
+ "bdi_goal=%lu bdi_dirty=%lu avg_dirty=%lu "
+ "base_bw=%lu task_bw=%lu "
+ "dirtied=%u dirtied_pause=%u "
+ "period=%lu think=%ld pause=%ld paused=%lu",
+ __entry->bdi,
+ __entry->limit,
+ __entry->goal,
+ __entry->dirty,
+ __entry->bdi_goal,
+ __entry->bdi_dirty,
+ __entry->avg_dirty,
+ __entry->base_bw, /* base throttle bandwidth */
+ __entry->task_bw, /* task throttle bandwidth */
+ __entry->dirtied,
+ __entry->dirtied_pause,
+ __entry->period, /* ms */
+ __entry->think, /* ms */
+ __entry->pause, /* ms */
+ __entry->paused /* ms */
+ )
+);
+
DECLARE_EVENT_CLASS(writeback_congest_waited_template,
TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
--- linux-next.orig/mm/page-writeback.c 2011-04-16 11:28:27.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-04-16 11:28:28.000000000 +0800
@@ -1163,6 +1163,16 @@ static void balance_dirty_pages(struct a
* it may be a light dirtier.
*/
if (unlikely(-pause < HZ*10)) {
+ trace_balance_dirty_pages(bdi,
+ dirty_thresh,
+ nr_dirty,
+ bdi_dirty,
+ base_bw,
+ bw,
+ pages_dirtied,
+ period,
+ pause,
+ start_time);
if (-pause > HZ/2) {
current->paused_when = now;
current->nr_dirtied = 0;
@@ -1176,6 +1186,16 @@ static void balance_dirty_pages(struct a
pause = min(pause, pause_max);
pause:
+ trace_balance_dirty_pages(bdi,
+ dirty_thresh,
+ nr_dirty,
+ bdi_dirty,
+ base_bw,
+ bw,
+ pages_dirtied,
+ period,
+ pause,
+ start_time);
current->paused_when = now;
__set_current_state(TASK_UNINTERRUPTIBLE);
io_schedule_timeout(pause);
--
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