[PATCH 37/45] writeback: use timestamp to indicate dirty exceeded

[Wu Fengguang]

When there are only one (or several) dirtiers, dirty_exceeded is always
(or mostly) off. Converting to timestamp avoids this problem. It helps
to use smaller write_chunk for smoother throttling.

We'll lower ratelimit if saw dirty exceeded in the last 1 second.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---

Before patch, the wait time in balance_dirty_pages() are ~200ms:

[ 1093.397700] write_bandwidth: comm=swapper pages=1536 time=204ms
[ 1093.594319] write_bandwidth: comm=swapper pages=1536 time=196ms
[ 1093.796642] write_bandwidth: comm=swapper pages=1536 time=200ms

After patch, ~25ms:

[   90.261339] write_bandwidth: comm=swapper pages=192 time=20ms
[   90.293168] write_bandwidth: comm=swapper pages=192 time=24ms
[   90.323853] write_bandwidth: comm=swapper pages=192 time=24ms
[   90.354510] write_bandwidth: comm=swapper pages=192 time=28ms
[   90.389890] write_bandwidth: comm=swapper pages=192 time=28ms
[   90.421787] write_bandwidth: comm=swapper pages=192 time=24ms

 include/linux/backing-dev.h |    2 +-
 mm/backing-dev.c            |    1 -
 mm/page-writeback.c         |    9 +++------
 3 files changed, 4 insertions(+), 8 deletions(-)

--- linux.orig/mm/page-writeback.c	2009-10-07 14:32:43.000000000 +0800
+++ linux/mm/page-writeback.c	2009-10-07 14:32:46.000000000 +0800
@@ -523,14 +523,10 @@ static void balance_dirty_pages(struct a
 		goto out;
 
 throttle:
-	if (!bdi->dirty_exceeded)
-		bdi->dirty_exceeded = 1;
+	bdi->dirty_exceed_time = jiffies;
 
 	bdi_writeback_wait(bdi, write_chunk);
 
-	if (bdi->dirty_exceeded)
-		bdi->dirty_exceeded = 0;
-
 out:
 	/*
 	 * In laptop mode, we wait until hitting the higher threshold before
@@ -578,7 +574,8 @@ void balance_dirty_pages_ratelimited_nr(
 	unsigned long *p;
 
 	ratelimit = ratelimit_pages;
-	if (mapping->backing_dev_info->dirty_exceeded)
+	if (jiffies - mapping->backing_dev_info->dirty_exceed_time <
+							(unsigned long) HZ)
 		ratelimit >>= 3;
 
 	/*
--- linux.orig/include/linux/backing-dev.h	2009-10-07 14:31:53.000000000 +0800
+++ linux/include/linux/backing-dev.h	2009-10-07 14:32:46.000000000 +0800
@@ -74,7 +74,7 @@ struct backing_dev_info {
 	struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];
 
 	struct prop_local_percpu completions;
-	int dirty_exceeded;
+	unsigned long dirty_exceed_time;
 
 	unsigned int min_ratio;
 	unsigned int max_ratio, max_prop_frac;
--- linux.orig/mm/backing-dev.c	2009-10-07 14:31:53.000000000 +0800
+++ linux/mm/backing-dev.c	2009-10-07 14:32:46.000000000 +0800
@@ -661,7 +661,6 @@ int bdi_init(struct backing_dev_info *bd
 			goto err;
 	}
 
-	bdi->dirty_exceeded = 0;
 	err = prop_local_init_percpu(&bdi->completions);
 
 	if (err) {


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