From 57fc978cfb61ed40a7bbfe5a569359159ba31abd Mon Sep 17 00:00:00 2001 From: Wu Fengguang Date: Sat, 11 Jun 2011 19:32:32 -0600 Subject: writeback: control dirty pause time 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 --- mm/page-writeback.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index cc351e6f9ed..6a8bb693b42 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -1086,6 +1086,10 @@ static void balance_dirty_pages(struct address_space *mapping, 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; -- cgit v1.2.3-70-g09d2