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SubjectRe: [PATCH 5/5] writeback: IO-less balance_dirty_pages()
On Sat, Aug 06, 2011 at 04:44:52PM +0800, Wu Fengguang wrote:
> As proposed by Chris, Dave and Jan, don't start foreground writeback IO
> inside balance_dirty_pages(). Instead, simply let it idle sleep for some
> time to throttle the dirtying task. In the mean while, kick off the
> per-bdi flusher thread to do background writeback IO.
>
> RATIONALS
> =========
>
> - disk seeks on concurrent writeback of multiple inodes (Dave Chinner)
>
> If every thread doing writes and being throttled start foreground
> writeback, it leads to N IO submitters from at least N different
> inodes at the same time, end up with N different sets of IO being
> issued with potentially zero locality to each other, resulting in
> much lower elevator sort/merge efficiency and hence we seek the disk
> all over the place to service the different sets of IO.
> OTOH, if there is only one submission thread, it doesn't jump between
> inodes in the same way when congestion clears - it keeps writing to
> the same inode, resulting in large related chunks of sequential IOs
> being issued to the disk. This is more efficient than the above
> foreground writeback because the elevator works better and the disk
> seeks less.
>
> - lock contention and cache bouncing on concurrent IO submitters (Dave Chinner)
>
> With this patchset, the fs_mark benchmark on a 12-drive software RAID0 goes
> from CPU bound to IO bound, freeing "3-4 CPUs worth of spinlock contention".
>
> * "CPU usage has dropped by ~55%", "it certainly appears that most of
> the CPU time saving comes from the removal of contention on the
> inode_wb_list_lock" (IMHO at least 10% comes from the reduction of
> cacheline bouncing, because the new code is able to call much less
> frequently into balance_dirty_pages() and hence access the global
> page states)
>
> * the user space "App overhead" is reduced by 20%, by avoiding the
> cacheline pollution by the complex writeback code path
>
> * "for a ~5% throughput reduction", "the number of write IOs have
> dropped by ~25%", and the elapsed time reduced from 41:42.17 to
> 40:53.23.
>
> * 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.
>
> - IO size too small for fast arrays and too large for slow USB sticks
>
> The write_chunk used by current balance_dirty_pages() cannot be
> directly set to some large value (eg. 128MB) for better IO efficiency.
> Because it could lead to more than 1 second user perceivable stalls.
> Even the current 4MB write size may be too large for slow USB sticks.
> The fact that balance_dirty_pages() starts IO on itself couples the
> IO size to wait time, which makes it hard to do suitable IO size while
> keeping the wait time under control.
>
> Now it's possible to increase writeback chunk size proportional to the
> disk bandwidth. In a simple test of 50 dd's on XFS, 1-HDD, 3GB ram,
> the larger writeback size dramatically reduces the seek count to 1/10
> (far beyond my expectation) and improves the write throughput by 24%.
>
> - long block time in balance_dirty_pages() hurts desktop responsiveness
>
> Many of us may have the experience: it often takes a couple of seconds
> or even long time to stop a heavy writing dd/cp/tar command with
> Ctrl-C or "kill -9".
>
> - IO pipeline broken by bumpy write() progress
>
> There are a broad class of "loop {read(buf); write(buf);}" applications
> whose read() pipeline will be under-utilized or even come to a stop if
> the write()s have long latencies _or_ don't progress in a constant rate.
> The current threshold based throttling inherently transfers the large
> low level IO completion fluctuations to bumpy application write()s,
> and further deteriorates with increasing number of dirtiers and/or bdi's.
>
> For example, when doing 50 dd's + 1 remote rsync to an XFS partition,
> the rsync progresses very bumpy in legacy kernel, and throughput is
> improved by 67% by this patchset. (plus the larger write chunk size,
> it will be 93% speedup).
>
> The new rate based throttling can support 1000+ dd's with excellent
> smoothness, low latency and low overheads.
>
> For the above reasons, it's much better to do IO-less and low latency
> pauses in balance_dirty_pages().
>
> Jan Kara, Dave Chinner and me explored the scheme to let
> balance_dirty_pages() wait for enough writeback IO completions to
> safeguard the dirty limit. However it's found to have two problems:
>
> - in large NUMA systems, the per-cpu counters may have big accounting
> errors, leading to big throttle wait time and jitters.
>
> - NFS may kill large amount of unstable pages with one single COMMIT.
> Because NFS server serves COMMIT with expensive fsync() IOs, it is
> desirable to delay and reduce the number of COMMITs. So it's not
> likely to optimize away such kind of bursty IO completions, and the
> resulted large (and tiny) stall times in IO completion based throttling.
>
> So here is a pause time oriented approach, which tries to control the
> pause time in each balance_dirty_pages() invocations, by controlling
> the number of pages dirtied before calling balance_dirty_pages(), for
> smooth and efficient dirty throttling:
>
> - avoid useless (eg. zero pause time) balance_dirty_pages() calls
> - avoid too small pause time (less than 4ms, which burns CPU power)
> - avoid too large pause time (more than 200ms, which hurts responsiveness)
> - avoid big fluctuations of pause times

I definitely agree that too small pauses must be avoided. However, I
don't understand very well from the code how the minimum sleep time is
regulated.

I've added a simple tracepoint (see below) to monitor the pause times in
balance_dirty_pages().

Sometimes I see very small pause time if I set a low dirty threshold
(<=32MB).

Example:

# echo $((16 * 1024 * 1024)) > /proc/sys/vm/dirty_bytes
# iozone -A >/dev/null &
# cat /sys/kernel/debug/tracing/trace_pipe
...
iozone-2075 [001] 380.604961: writeback_dirty_throttle: 1
iozone-2075 [001] 380.605966: writeback_dirty_throttle: 2
iozone-2075 [001] 380.608405: writeback_dirty_throttle: 0
iozone-2075 [001] 380.608980: writeback_dirty_throttle: 1
iozone-2075 [001] 380.609952: writeback_dirty_throttle: 1
iozone-2075 [001] 380.610952: writeback_dirty_throttle: 2
iozone-2075 [001] 380.612662: writeback_dirty_throttle: 0
iozone-2075 [000] 380.613799: writeback_dirty_throttle: 1
iozone-2075 [000] 380.614771: writeback_dirty_throttle: 1
iozone-2075 [000] 380.615767: writeback_dirty_throttle: 2
...
BTW, I can see this behavior only in the first minute while iozone is
running. Ater ~1min things seem to get stable (sleeps are usually
between 50ms and 200ms).

I wonder if we shouldn't add an explicit check also for the minimum
sleep time.

Thanks,
-Andrea

Signed-off-by: Andrea Righi <andrea@betterlinux.com>
---
include/trace/events/writeback.h | 12 ++++++++++++
mm/page-writeback.c | 1 +
2 files changed, 13 insertions(+), 0 deletions(-)
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 9c2cc8a..22b04b9 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -78,6 +78,18 @@ TRACE_EVENT(writeback_pages_written,
TP_printk("%ld", __entry->pages)
);

+TRACE_EVENT(writeback_dirty_throttle,
+ TP_PROTO(unsigned long sleep),
+ TP_ARGS(sleep),
+ TP_STRUCT__entry(
+ __field(unsigned long, sleep)
+ ),
+ TP_fast_assign(
+ __entry->sleep = sleep;
+ ),
+ TP_printk("%u", jiffies_to_msecs(__entry->sleep))
+);
+
DECLARE_EVENT_CLASS(writeback_class,
TP_PROTO(struct backing_dev_info *bdi),
TP_ARGS(bdi),
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index a998931..e5a2664 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -889,6 +889,7 @@ static void balance_dirty_pages(struct address_space *mapping,
pause = min_t(unsigned long, pause, MAX_PAUSE);

pause:
+ trace_writeback_dirty_throttle(pause);
__set_current_state(TASK_UNINTERRUPTIBLE);
io_schedule_timeout(pause);

>
> It can control pause times at will. The default policy will be to do
> ~10ms pauses in 1-dd case, and increase to ~100ms in 1000-dd case.
>
> BEHAVIOR CHANGE
> ===============
>
> (1) dirty threshold
>
> Users will notice that the applications will get throttled once crossing
> the global (background + dirty)/2=15% threshold, and then balanced around
> 17.5%. Before patch, the behavior is to just throttle it at 20% dirtyable
> memory in 1-dd case.
>
> Since the task will be soft throttled earlier than before, it may be
> perceived by end users as performance "slow down" if his application
> happens to dirty more than 15% dirtyable memory.
>
> (2) smoothness/responsiveness
>
> Users will notice a more responsive system during heavy writeback.
> "killall dd" will take effect instantly.
>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> include/trace/events/writeback.h | 24 ----
> mm/page-writeback.c | 142 +++++++----------------------
> 2 files changed, 37 insertions(+), 129 deletions(-)
>
> --- linux-next.orig/mm/page-writeback.c 2011-08-06 11:17:26.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-08-06 16:16:30.000000000 +0800
> @@ -242,50 +242,6 @@ static void bdi_writeout_fraction(struct
> numerator, denominator);
> }
>
> -static inline void task_dirties_fraction(struct task_struct *tsk,
> - long *numerator, long *denominator)
> -{
> - prop_fraction_single(&vm_dirties, &tsk->dirties,
> - numerator, denominator);
> -}
> -
> -/*
> - * task_dirty_limit - scale down dirty throttling threshold for one task
> - *
> - * task specific dirty limit:
> - *
> - * dirty -= (dirty/8) * p_{t}
> - *
> - * To protect light/slow dirtying tasks from heavier/fast ones, we start
> - * throttling individual tasks before reaching the bdi dirty limit.
> - * Relatively low thresholds will be allocated to heavy dirtiers. So when
> - * dirty pages grow large, heavy dirtiers will be throttled first, which will
> - * effectively curb the growth of dirty pages. Light dirtiers with high enough
> - * dirty threshold may never get throttled.
> - */
> -#define TASK_LIMIT_FRACTION 8
> -static unsigned long task_dirty_limit(struct task_struct *tsk,
> - unsigned long bdi_dirty)
> -{
> - long numerator, denominator;
> - unsigned long dirty = bdi_dirty;
> - u64 inv = dirty / TASK_LIMIT_FRACTION;
> -
> - task_dirties_fraction(tsk, &numerator, &denominator);
> - inv *= numerator;
> - do_div(inv, denominator);
> -
> - dirty -= inv;
> -
> - return max(dirty, bdi_dirty/2);
> -}
> -
> -/* Minimum limit for any task */
> -static unsigned long task_min_dirty_limit(unsigned long bdi_dirty)
> -{
> - return bdi_dirty - bdi_dirty / TASK_LIMIT_FRACTION;
> -}
> -
> /*
> *
> */
> @@ -855,24 +811,28 @@ static unsigned long ratelimit_pages(uns
> * perform some writeout.
> */
> static void balance_dirty_pages(struct address_space *mapping,
> - unsigned long write_chunk)
> + unsigned long pages_dirtied)
> {
> - unsigned long nr_reclaimable, bdi_nr_reclaimable;
> + unsigned long nr_reclaimable;
> unsigned long nr_dirty; /* = file_dirty + writeback + unstable_nfs */
> unsigned long bdi_dirty;
> unsigned long background_thresh;
> unsigned long dirty_thresh;
> unsigned long bdi_thresh;
> - unsigned long task_bdi_thresh;
> - unsigned long min_task_bdi_thresh;
> - unsigned long pages_written = 0;
> - unsigned long pause = 1;
> + unsigned long pause = 0;
> bool dirty_exceeded = false;
> - bool clear_dirty_exceeded = true;
> + unsigned long bw;
> + unsigned long base_bw;
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> unsigned long start_time = jiffies;
>
> for (;;) {
> + /*
> + * Unstable writes are a feature of certain networked
> + * filesystems (i.e. NFS) in which data may have been
> + * written to the server's write cache, but has not yet
> + * been flushed to permanent storage.
> + */
> nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> global_page_state(NR_UNSTABLE_NFS);
> nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
> @@ -888,8 +848,6 @@ static void balance_dirty_pages(struct a
> break;
>
> bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
> - min_task_bdi_thresh = task_min_dirty_limit(bdi_thresh);
> - task_bdi_thresh = task_dirty_limit(current, bdi_thresh);
>
> /*
> * In order to avoid the stacked BDI deadlock we need
> @@ -901,56 +859,38 @@ static void balance_dirty_pages(struct a
> * actually dirty; with m+n sitting in the percpu
> * deltas.
> */
> - if (task_bdi_thresh < 2 * bdi_stat_error(bdi)) {
> - bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> - bdi_dirty = bdi_nr_reclaimable +
> + if (bdi_thresh < 2 * bdi_stat_error(bdi))
> + bdi_dirty = bdi_stat_sum(bdi, BDI_RECLAIMABLE) +
> bdi_stat_sum(bdi, BDI_WRITEBACK);
> - } else {
> - bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> - bdi_dirty = bdi_nr_reclaimable +
> + else
> + bdi_dirty = bdi_stat(bdi, BDI_RECLAIMABLE) +
> bdi_stat(bdi, BDI_WRITEBACK);
> - }
>
> - /*
> - * The bdi thresh is somehow "soft" limit derived from the
> - * global "hard" limit. The former helps to prevent heavy IO
> - * bdi or process from holding back light ones; The latter is
> - * the last resort safeguard.
> - */
> - dirty_exceeded = (bdi_dirty > task_bdi_thresh) ||
> + dirty_exceeded = (bdi_dirty > bdi_thresh) ||
> (nr_dirty > dirty_thresh);
> - clear_dirty_exceeded = (bdi_dirty <= min_task_bdi_thresh) &&
> - (nr_dirty <= dirty_thresh);
> -
> - if (!dirty_exceeded)
> - break;
> -
> - if (!bdi->dirty_exceeded)
> + if (dirty_exceeded && !bdi->dirty_exceeded)
> bdi->dirty_exceeded = 1;
>
> bdi_update_bandwidth(bdi, dirty_thresh, nr_dirty,
> bdi_thresh, bdi_dirty, start_time);
>
> - /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
> - * Unstable writes are a feature of certain networked
> - * filesystems (i.e. NFS) in which data may have been
> - * written to the server's write cache, but has not yet
> - * been flushed to permanent storage.
> - * Only move pages to writeback if this bdi is over its
> - * threshold otherwise wait until the disk writes catch
> - * up.
> - */
> - trace_balance_dirty_start(bdi);
> - if (bdi_nr_reclaimable > task_bdi_thresh) {
> - pages_written += writeback_inodes_wb(&bdi->wb,
> - write_chunk);
> - trace_balance_dirty_written(bdi, pages_written);
> - if (pages_written >= write_chunk)
> - break; /* We've done our duty */
> + if (unlikely(!writeback_in_progress(bdi)))
> + bdi_start_background_writeback(bdi);
> +
> + base_bw = bdi->dirty_ratelimit;
> + bw = bdi_position_ratio(bdi, dirty_thresh, nr_dirty,
> + bdi_thresh, bdi_dirty);
> + if (unlikely(bw == 0)) {
> + pause = MAX_PAUSE;
> + goto pause;
> }
> + bw = (u64)base_bw * bw >> BANDWIDTH_CALC_SHIFT;
> + pause = (HZ * pages_dirtied + bw / 2) / (bw | 1);
> + pause = min(pause, MAX_PAUSE);
> +
> +pause:
> __set_current_state(TASK_UNINTERRUPTIBLE);
> io_schedule_timeout(pause);
> - trace_balance_dirty_wait(bdi);
>
> dirty_thresh = hard_dirty_limit(dirty_thresh);
> /*
> @@ -960,8 +900,7 @@ static void balance_dirty_pages(struct a
> * (b) the pause time limit makes the dirtiers more responsive.
> */
> if (nr_dirty < dirty_thresh +
> - dirty_thresh / DIRTY_MAXPAUSE_AREA &&
> - time_after(jiffies, start_time + MAX_PAUSE))
> + dirty_thresh / DIRTY_MAXPAUSE_AREA)
> break;
> /*
> * pass-good area. When some bdi gets blocked (eg. NFS server
> @@ -974,18 +913,9 @@ static void balance_dirty_pages(struct a
> dirty_thresh / DIRTY_PASSGOOD_AREA &&
> bdi_dirty < bdi_thresh)
> break;
> -
> - /*
> - * Increase the delay for each loop, up to our previous
> - * default of taking a 100ms nap.
> - */
> - pause <<= 1;
> - if (pause > HZ / 10)
> - pause = HZ / 10;
> }
>
> - /* Clear dirty_exceeded flag only when no task can exceed the limit */
> - if (clear_dirty_exceeded && bdi->dirty_exceeded)
> + if (!dirty_exceeded && bdi->dirty_exceeded)
> bdi->dirty_exceeded = 0;
>
> current->nr_dirtied = 0;
> @@ -1002,8 +932,10 @@ static void balance_dirty_pages(struct a
> * In normal mode, we start background writeout at the lower
> * background_thresh, to keep the amount of dirty memory low.
> */
> - if ((laptop_mode && pages_written) ||
> - (!laptop_mode && (nr_reclaimable > background_thresh)))
> + if (laptop_mode)
> + return;
> +
> + if (nr_reclaimable > background_thresh)
> bdi_start_background_writeback(bdi);
> }
>
> --- linux-next.orig/include/trace/events/writeback.h 2011-08-06 11:08:34.000000000 +0800
> +++ linux-next/include/trace/events/writeback.h 2011-08-06 11:17:29.000000000 +0800
> @@ -104,30 +104,6 @@ DEFINE_WRITEBACK_EVENT(writeback_bdi_reg
> DEFINE_WRITEBACK_EVENT(writeback_bdi_unregister);
> DEFINE_WRITEBACK_EVENT(writeback_thread_start);
> DEFINE_WRITEBACK_EVENT(writeback_thread_stop);
> -DEFINE_WRITEBACK_EVENT(balance_dirty_start);
> -DEFINE_WRITEBACK_EVENT(balance_dirty_wait);
> -
> -TRACE_EVENT(balance_dirty_written,
> -
> - TP_PROTO(struct backing_dev_info *bdi, int written),
> -
> - TP_ARGS(bdi, written),
> -
> - TP_STRUCT__entry(
> - __array(char, name, 32)
> - __field(int, written)
> - ),
> -
> - TP_fast_assign(
> - strncpy(__entry->name, dev_name(bdi->dev), 32);
> - __entry->written = written;
> - ),
> -
> - TP_printk("bdi %s written %d",
> - __entry->name,
> - __entry->written
> - )
> -);
>
> DECLARE_EVENT_CLASS(wbc_class,
> TP_PROTO(struct writeback_control *wbc, struct backing_dev_info *bdi),
>


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