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 | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
--- linux-next.orig/mm/page-writeback.c 2011-10-03 17:35:57.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-10-03 17:39:27.000000000 +0800
@@ -1086,6 +1086,10 @@ static void balance_dirty_pages(struct a
task_ratelimit = (u64)dirty_ratelimit *
pos_ratio >> RATELIMIT_CALC_SHIFT;
pause = (HZ * pages_dirtied) / (task_ratelimit | 1);
+ if (unlikely(pause <= 0)) {
+ pause = 1; /* avoid resetting nr_dirtied_pause below */
+ break;
+ }
pause = min(pause, max_pause);
pause:
@@ -1107,7 +1111,21 @@ pause:
bdi->dirty_exceeded = 0;
current->nr_dirtied = 0;
- current->nr_dirtied_pause = dirty_poll_interval(nr_dirty, dirty_thresh);
+ if (pause == 0) { /* in freerun area */
+ current->nr_dirtied_pause =
+ dirty_poll_interval(nr_dirty, dirty_thresh);
+ } else if (pause <= 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 / 2) / HZ,
+ pages_dirtied / 4,
+ pages_dirtied - pages_dirtied / 8);
+ }
if (writeback_in_progress(bdi))
return;
--
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