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    Subject[PATCH 12/35] writeback: scale down max throttle bandwidth on concurrent dirtiers
    This will noticeably reduce the fluctuaions of pause time when there are
    100+ concurrent dirtiers.

    The more parallel dirtiers (1 dirtier => 4 dirtiers), the smaller
    bandwidth each dirtier will share (bdi_bandwidth => bdi_bandwidth/4),
    the less gap to the dirty limit ((C-A) => (C-B)), the less stable the
    pause time will be (given the same fluctuation of bdi_dirty).

    For example, if A drifts to A', its pause time may drift from 5ms to
    6ms, while B to B' may drift from 50ms to 90ms. It's much larger
    fluctuations in relative ratio as well as absolute time.

    Fig.1 before patch, gap (C-B) is too low to get smooth pause time

    throttle_bandwidth_A = bdi_bandwidth .........o
    | o <= A'
    | o
    | o
    | o
    | o
    throttle_bandwidth_B = bdi_bandwidth / 4 .....|...........o
    | | o <= B'
    ----------------------------------------------+-----------+---o
    A B C

    The solution is to lower the slope of the throttle line accordingly,
    which makes B stabilize at some point more far away from C.

    Fig.2 after patch

    throttle_bandwidth_A = bdi_bandwidth .........o
    | o <= A'
    | o
    | o
    lowered max throttle bandwidth for B ===> * o
    | * o
    throttle_bandwidth_B = bdi_bandwidth / 4 .............* o
    | | * o
    ----------------------------------------------+-------+-------o
    A B C

    Note that C is actually different points for 1-dirty and 4-dirtiers
    cases, but for easy graphing, we move them together.

    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/page-writeback.c | 16 +++++++++++++---
    1 file changed, 13 insertions(+), 3 deletions(-)

    --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:14.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2010-12-13 21:46:15.000000000 +0800
    @@ -587,6 +587,7 @@ static void balance_dirty_pages(struct a
    unsigned long background_thresh;
    unsigned long dirty_thresh;
    unsigned long bdi_thresh;
    + unsigned long task_thresh;
    unsigned long long bw;
    unsigned long period;
    unsigned long pause = 0;
    @@ -616,7 +617,7 @@ static void balance_dirty_pages(struct a
    break;

    bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh, nr_dirty);
    - bdi_thresh = task_dirty_limit(current, bdi_thresh);
    + task_thresh = task_dirty_limit(current, bdi_thresh);

    /*
    * In order to avoid the stacked BDI deadlock we need
    @@ -638,14 +639,23 @@ static void balance_dirty_pages(struct a

    bdi_update_bandwidth(bdi, start_time, bdi_dirty, bdi_thresh);

    - if (bdi_dirty >= bdi_thresh || nr_dirty > dirty_thresh) {
    + if (bdi_dirty >= task_thresh || nr_dirty > dirty_thresh) {
    pause = MAX_PAUSE;
    goto pause;
    }

    + /*
    + * When bdi_dirty grows closer to bdi_thresh, it indicates more
    + * concurrent dirtiers. Proportionally lower the max throttle
    + * bandwidth. This will resist bdi_dirty from approaching to
    + * close to task_thresh, and help reduce fluctuations of pause
    + * time when there are lots of dirtiers.
    + */
    bw = bdi->write_bandwidth;
    -
    bw = bw * (bdi_thresh - bdi_dirty);
    + do_div(bw, bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1);
    +
    + bw = bw * (task_thresh - bdi_dirty);
    do_div(bw, bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1);

    period = HZ * pages_dirtied / ((unsigned long)bw + 1) + 1;



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