[PATCH 14/18] writeback: control dirty pause time

[Wu Fengguang]

The dirty pause time shall ultimately be controlled by adjusting
nr_dirtied_pause, since there is relationship

	pause = pages_dirtied / task_ratelimit

Assuming

	pages_dirtied ~= nr_dirtied_pause
	task_ratelimit ~= dirty_ratelimit

We get

	nr_dirtied_pause ~= dirty_ratelimit * desired_pause

Here dirty_ratelimit is preferred over task_ratelimit because it's
more stable.

It's also important to limit possible large transitional errors:

- bw is changing quickly
- pages_dirtied << nr_dirtied_pause on entering dirty exceeded area
- pages_dirtied >> nr_dirtied_pause on btrfs (to be improved by a
  separate fix, but still expect non-trivial errors)

So we end up using the above formula inside clamp_val().

The best test case for this code is to run 100 "dd bs=4M" tasks on
btrfs and check its pause time distribution.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/page-writeback.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

--- linux-next.orig/mm/page-writeback.c	2011-08-29 19:08:43.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-08-29 19:08:44.000000000 +0800
@@ -1193,7 +1193,20 @@ pause:
 	if (!dirty_exceeded && bdi->dirty_exceeded)
 		bdi->dirty_exceeded = 0;
 
-	current->nr_dirtied_pause = dirty_poll_interval(nr_dirty, dirty_thresh);
+	if (pause == 0)
+		current->nr_dirtied_pause =
+				dirty_poll_interval(nr_dirty, dirty_thresh);
+	else if (period <= max_pause / 4 &&
+		 pages_dirtied >= current->nr_dirtied_pause)
+		current->nr_dirtied_pause = clamp_val(
+					dirty_ratelimit * (max_pause / 2) / HZ,
+					pages_dirtied + pages_dirtied / 8,
+					pages_dirtied * 4);
+	else if (pause >= max_pause)
+		current->nr_dirtied_pause = 1 | clamp_val(
+					dirty_ratelimit * (max_pause * 3/8)/HZ,
+					pages_dirtied / 4,
+					pages_dirtied * 7/8);
 
 	if (writeback_in_progress(bdi))
 		return;


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