Re: [PATCH] mm, memcg: reclaim more aggressively before high allocator throttling

[Chris Down]

Michal Hocko writes:
> > We send a simple bug fix: bring this instance of reclaim in line with
> > how everybody else is using the reclaim API, to meet the semantics as
> > they are intendend and documented.
>
> Here is where we are not on the same page though. Once you have identified
> that the main problem is that the reclaim fails too early to meet the
> target then the fix would be to enforce that target. I have asked why
> this hasn't been done and haven't got any real answer for that. Instead
> what you call "a simple bug fix" has larger consequences which are not
> really explained in the changelog and they are also not really trivial
> to see. If the changelog explicitly stated that the proportional memory
> reclaim is not sufficient because XYZ and the implementation has been
> changed to instead meet the high limit target then this would be a
> completely different story and I believe we could have saved some
> discussion.

I agree that the changelog can be made more clear. Any objection if I send v2 
with changelog changes to that effect, then? :-)

> > And somehow this is controversial, and we're just changing around user
> > promises as we see fit for our particular usecase?
> >
> > I don't even understand how the supposed alternate semantics you read
> > between the lines in the documentation would make for a useful
> > feature: It may fail to contain a group of offending tasks to the
> > configured limit, but it will be fair to those tasks while doing so?
> >
> > > But if your really want to push this through then let's do it
> > > properly at least. memcg->memcg_nr_pages_over_high has only very
> > > vague meaning if the reclaim target is the high limit.
> >
> > task->memcg_nr_pages_over_high is not vague, it's a best-effort
> > mechanism to distribute fairness. It's the current task's share of the
> > cgroup's overage, and it allows us in the majority of situations to
> > distribute reclaim work and sleeps in proportion to how much the task
> > is actually at fault.
>
> Agreed. But this stops being the case as soon as the reclaim target has
> been reached and new reclaim attempts are enforced because the memcg is
> still above the high limit. Because then you have a completely different
> reclaim target - get down to the limit. This would be especially visible
> with a large memcg_nr_pages_over_high which could even lead to an over
> reclaim.

We actually over reclaim even before this patch -- this patch doesn't bring 
much new in that regard.

Tracing try_to_free_pages for a cgroup at the memory.high threshold shows that 
before this change, we sometimes even reclaim on the order of twice the number 
of pages requested. For example, I see cases where we requested 1000 pages to 
be reclaimed, but end up reclaiming 2000 in a single reclaim attempt.