Hi Greg,
On Wed, Feb 01, 2012 at 12:24:25PM -0800, Greg Thelen wrote:
> On Tue, Jan 31, 2012 at 4:55 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > 4. dirty ratio
> > In the last year, patches were posted but not merged. I'd like to hear
> > works on this area.
>
> I would like to attend to discuss this topic. I have not had much time to work
> on this recently, but should be able to focus more on this soon. The
> IO less writeback changes require some redesign and may allow for a
> simpler implementation of mem_cgroup_balance_dirty_pages().
> Maintaining a per container dirty page counts, ratios, and limits is
> fairly easy, but integration with writeback is the challenge. My big
> questions are for writeback people:
> 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
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