writeback: control dirty pause time (57fc978c) · Commits · 戴 / test

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

+19 −1

Original line number	Diff line number	Diff line
		@@ -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;

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