On Thu 28-05-20 17:48:48, Chris Down wrote: > 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? :-) Yes, please. And I would highly appreciate to have the above addressed. So that we do not have to really scratch heads why a particular design decision has been made and argue what was the thinking behind. > > > 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. This is interesting and worth looking into. I am aware that we can reclaim potentially much more pages during the icache reclaim and that there was a heated discussion without any fix merged in the end IIRC. Do you have any details? -- Michal Hocko SUSE Labs
