[PATCH 16/27] writeback: smoothed global/bdi dirty pages

[Wu Fengguang]

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.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 include/linux/backing-dev.h |    2 +
 mm/backing-dev.c            |    3 +
 mm/page-writeback.c         |   62 ++++++++++++++++++++++++++++++++++
 3 files changed, 67 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2011-03-03 14:44:07.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-03-03 14:44:10.000000000 +0800
@@ -472,6 +472,64 @@ 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) >> 3;
+		goto update;
+	}
+
+	/*
+	 * dirty pages are departing downwards, follow down
+	 */
+	if (avg > old && old >= dirty) {
+		avg -= (avg - old) >> 3;
+		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                       .       .
+	 *                                                   .       .
+	 *              .         .         .         .     .       .
+	 *      ---------------------------------------    .       .
+	 *            .         .         .         .     .       .
+	 *           .         .         .         .     .       .
+	 *          .         .         .         .     .       .
+	 *         .         .         .         .     .       .
+	 *        .         .         .         .
+	 *       .         .         .         .      (flucuated)
+	 *      .         .         .         .
+	 *     .         .         .         .
+	 *
+	 * @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)
@@ -537,6 +595,10 @@ void bdi_update_bandwidth(struct backing
 		goto unlock;
 
 	__bdi_update_write_bandwidth(bdi, elapsed, written);
+	if (thresh) {
+		bdi_update_dirty_smooth(bdi, bdi_dirty);
+		bdi_update_dirty_smooth(&default_backing_dev_info, dirty);
+	}
 
 snapshot:
 	bdi->written_stamp = written;
--- linux-next.orig/include/linux/backing-dev.h	2011-03-03 14:44:07.000000000 +0800
+++ linux-next/include/linux/backing-dev.h	2011-03-03 14:44:10.000000000 +0800
@@ -79,6 +79,8 @@ struct backing_dev_info {
 	unsigned long written_stamp;
 	unsigned long write_bandwidth;
 	unsigned long avg_bandwidth;
+	unsigned long avg_dirty;
+	unsigned long old_dirty;
 
 	struct prop_local_percpu completions;
 	int dirty_exceeded;
--- linux-next.orig/mm/backing-dev.c	2011-03-03 14:44:07.000000000 +0800
+++ linux-next/mm/backing-dev.c	2011-03-03 14:44:10.000000000 +0800
@@ -675,6 +675,9 @@ int bdi_init(struct backing_dev_info *bd
 	bdi->write_bandwidth = INIT_BW;
 	bdi->avg_bandwidth = INIT_BW;
 
+	bdi->avg_dirty = 0;
+	bdi->old_dirty = 0;
+
 	err = prop_local_init_percpu(&bdi->completions);
 
 	if (err) {


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