[PATCH 31/47] writeback: increase min pause time on concurrent dirtiers

Wu Fengguang
2010-12-13T06:49:47Z

[-- Attachment #1: writeback-min-pause-time-for-concurrent-dirtiers.patch --]
[-- Type: text/plain, Size: 2104 bytes --]

Target for >60ms pause time when there are 100+ heavy dirtiers per bdi.
(will average around 100ms given 200ms max pause time)

It's OK for 1 dd task doing 100MB/s to be throttle paused 100 times per
second. However when there are 100 tasks writing to the same disk,
That sums up to 100*100 balance_dirty_pages() calls per second and may
lead to massive cacheline bouncing on accessing the global page states
in NUMA machines. Even in single socket boxes, we easily see >10% CPU
time reduction by increasing the pause time.

CC: Dave Chinner <david@fromorbit.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/page-writeback.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

--- linux-next.orig/mm/page-writeback.c 2010-12-09 12:24:45.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-12-09 12:24:47.000000000 +0800
@@ -666,6 +666,27 @@ static unsigned long max_pause(unsigned
}

/*
+ * Scale up pause time for concurrent dirtiers in order to reduce CPU overheads.
+ * But ensure reasonably large [min_pause, max_pause] range size, so that
+ * nr_dirtied_pause (and hence future pause time) can stay reasonably stable.
+ */
+static unsigned long min_pause(struct backing_dev_info *bdi,
+ unsigned long max)
+{
+ unsigned long hi = ilog2(bdi->write_bandwidth);
+ unsigned long lo = ilog2(bdi->throttle_bandwidth);
+ unsigned long t;
+
+ if (lo >= hi)
+ return 1;
+
+ /* (N * 10ms) on 2^N concurrent tasks */
+ t = (hi - lo) * (10 * HZ) / 1024;
+
+ return clamp_val(t, 1, max / 2);
+}
+
+/*
* balance_dirty_pages() must be called by processes which are generating dirty
* data. It looks at the number of dirty pages in the machine and will force
* the caller to perform writeback if the system is over `vm_dirty_ratio'.
@@ -833,7 +854,7 @@ pause:

if (pause == 0 && nr_dirty < background_thresh)
current->nr_dirtied_pause = ratelimit_pages(bdi);
- else if (pause == 1)
+ else if (pause <= min_pause(bdi, pause_max))
current->nr_dirtied_pause += current->nr_dirtied_pause / 32 + 1;
else if (pause >= pause_max)
/*