Re: [PATCH 12/35] writeback: scale down max throttle bandwidth onconcurrent dirtiers

[Yan, Zheng]

On Mon, Dec 13, 2010 at 10:46 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 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@xxxxxxxxx>
> ---
>  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);

Maybe changing this line to "do_div(bw, task_thresh /
TASK_SOFT_DIRTY_LIMIT + 1);"
is more consistent.

Thanks
Yan, Zheng
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