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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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