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SubjectRe: [PATCH 14/18] writeback: control dirty pause time
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On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> plain text document attachment (max-pause-adaption)
> 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 | 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);
>

I very much prefer { } over multi line stmts, even if not strictly
needed.
I'm also not quite sure why pause==0 is a special case, also, do the two
other line segments connect on the transition point?


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