Re: [LSF/MM TOPIC] memcg topics.

[Greg Thelen <gthelen@google.com>]

On Wed, Feb 1, 2012 at 10:33 PM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> Hi Greg,
>
> On Wed, Feb 01, 2012 at 12:24:25PM -0800, Greg Thelen wrote:
>> 1. how to compute per-container pause based on bdi bandwidth, cgroup
>> dirty page usage.
>> 2. how to ensure that writeback will engage even if system and bdi are
>> below respective background dirty ratios, yet a memcg is above its bg
>> dirty limit.
>
> The solution to (1,2) would be something like this:
>
> --- linux-next.orig/mm/page-writeback.c 2012-02-02 14:13:45.000000000 +0800
> +++ linux-next/mm/page-writeback.c      2012-02-02 14:24:11.000000000 +0800
> @@ -654,6 +654,17 @@ static unsigned long bdi_position_ratio(
>        pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
>        pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
>
> +       if (memcg) {
> +               long long f;
> +               x = div_s64((memcg_setpoint - memcg_dirty) << RATELIMIT_CALC_SHIFT,
> +                           memcg_limit - memcg_setpoint + 1);
> +               f = x;
> +               f = f * x >> RATELIMIT_CALC_SHIFT;
> +               f = f * x >> RATELIMIT_CALC_SHIFT;
> +               f += 1 << RATELIMIT_CALC_SHIFT;
> +               pos_ratio = pos_ratio * f >> RATELIMIT_CALC_SHIFT;
> +       }
> +
>        /*
>         * We have computed basic pos_ratio above based on global situation. If
>         * the bdi is over/under its share of dirty pages, we want to scale
> @@ -1202,6 +1213,8 @@ static void balance_dirty_pages(struct a
>                freerun = dirty_freerun_ceiling(dirty_thresh,
>                                                background_thresh);
>                if (nr_dirty <= freerun) {
> +                       if (memcg && memcg_dirty > memcg_freerun)
> +                               goto start_writeback;
>                        current->dirty_paused_when = now;
>                        current->nr_dirtied = 0;
>                        current->nr_dirtied_pause =
> @@ -1209,6 +1222,7 @@ static void balance_dirty_pages(struct a
>                        break;
>                }
>
> +start_writeback:
>                if (unlikely(!writeback_in_progress(bdi)))
>                        bdi_start_background_writeback(bdi);
>
>
> That makes the minimal change to enforce per-memcg dirty ratio.
> It could result in a less stable control system, but should still
> be able to balance things out.
>
> Thanks,
> Fengguang

Thank you for the quick patch.  It looks promising.  I can imagine how
this would wake up background writeback.  But I am unsure how
background writeback will do anything.  It seems like
over_bground_thresh() would not necessarily see system or bdi dirty
usage over respective limits.  In previously posted memcg writeback
patches this involved an fs-writeback.c call to
mem_cgroups_over_bground_dirty_thresh() to check for memcg dirty limit
compliance.  Do you think we still need such a call out to memcg from
writeback?

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>