Messages in this thread Patch in this message | | | Date | Mon, 03 Oct 2011 21:42:37 +0800 | From | Wu Fengguang <> | Subject | [PATCH 09/11] writeback: control dirty pause time |
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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@intel.com> --- 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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