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Subject[PATCH 04/12] writeback: smoothed global/bdi dirty pages
Maintain a smoothed version of dirty pages for use in the throttle
bandwidth calculations.

default_backing_dev_info.avg_dirty holds the smoothed global dirty
pages.

The calculation favors smoothness rather than accuracy. It's non-sense
trying to track a much fluctuated value "accurately". And its users
don't really rely on it being accurate.

CC: larry <lantianyu1986@gmail.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/backing-dev.h | 2 +
mm/backing-dev.c | 3 +
mm/page-writeback.c | 66 ++++++++++++++++++++++++++++++++++
3 files changed, 71 insertions(+)
--- linux-next.orig/mm/page-writeback.c 2011-04-13 17:18:12.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-04-13 17:18:12.000000000 +0800
@@ -471,6 +471,64 @@ unsigned long bdi_dirty_limit(struct bac
return bdi_dirty;
}

+static void bdi_update_dirty_smooth(struct backing_dev_info *bdi,
+ unsigned long dirty)
+{
+ unsigned long avg = bdi->avg_dirty;
+ unsigned long old = bdi->old_dirty;
+
+ if (unlikely(!avg)) {
+ avg = dirty;
+ goto update;
+ }
+
+ /*
+ * dirty pages are departing upwards, follow up
+ */
+ if (avg < old && old <= dirty) {
+ avg += (old - avg) >> 2;
+ goto update;
+ }
+
+ /*
+ * dirty pages are departing downwards, follow down
+ */
+ if (avg > old && old >= dirty) {
+ avg -= (avg - old) >> 2;
+ goto update;
+ }
+
+ /*
+ * This can filter out one half unnecessary updates when bdi_dirty is
+ * fluctuating around the balance point, and is most effective on XFS,
+ * whose pattern is
+ * .
+ * [.] dirty [-] avg . .
+ * . .
+ * . . . . . .
+ * --------------------------------------- . .
+ * . . . . . .
+ * . . . . . .
+ * . . . . . .
+ * . . . . . .
+ * . . . .
+ * . . . . (fluctuated)
+ * . . . .
+ * . . . .
+ *
+ * @avg will remain flat at the cost of being biased towards high. In
+ * practice the error tend to be much smaller: thanks to more coarse
+ * grained fluctuations, @avg becomes the real average number for the
+ * last two rising lines of @dirty.
+ */
+ goto out;
+
+update:
+ bdi->avg_dirty = avg;
+out:
+ bdi->old_dirty = dirty;
+}
+
static void __bdi_update_write_bandwidth(struct backing_dev_info *bdi,
unsigned long elapsed,
unsigned long written)
@@ -535,6 +593,14 @@ void bdi_update_bandwidth(struct backing
if (elapsed <= HZ / 5)
goto unlock;

+ if (thresh &&
+ now - default_backing_dev_info.bw_time_stamp >= HZ / 5) {
+ bdi_update_dirty_smooth(&default_backing_dev_info, dirty);
+ default_backing_dev_info.bw_time_stamp = now;
+ }
+ if (thresh) {
+ bdi_update_dirty_smooth(bdi, bdi_dirty);
+ }
__bdi_update_write_bandwidth(bdi, elapsed, written);

snapshot:
--- linux-next.orig/include/linux/backing-dev.h 2011-04-13 17:18:12.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2011-04-13 17:18:12.000000000 +0800
@@ -77,6 +77,8 @@ struct backing_dev_info {
unsigned long written_stamp;
unsigned long write_bandwidth;
unsigned long avg_write_bandwidth;
+ unsigned long avg_dirty;
+ unsigned long old_dirty;

struct prop_local_percpu completions;
int dirty_exceeded;
--- linux-next.orig/mm/backing-dev.c 2011-04-13 17:18:12.000000000 +0800
+++ linux-next/mm/backing-dev.c 2011-04-13 17:18:12.000000000 +0800
@@ -669,6 +669,9 @@ int bdi_init(struct backing_dev_info *bd
bdi->write_bandwidth = INIT_BW;
bdi->avg_write_bandwidth = INIT_BW;

+ bdi->avg_dirty = 0;
+ bdi->old_dirty = 0;
+
err = prop_local_init_percpu(&bdi->completions);

if (err) {



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