Messages in this thread | | | Date | Thu, 28 May 2020 17:48:48 +0100 | From | Chris Down <> | Subject | Re: [PATCH] mm, memcg: reclaim more aggressively before high allocator throttling |
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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.
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