On Tue, Nov 12, 2019 at 10:45:44AM -0800, Suren Baghdasaryan wrote: > On Tue, Nov 12, 2019 at 9:45 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > On Sun, Nov 10, 2019 at 06:01:18PM -0800, Suren Baghdasaryan wrote: > > > On Thu, Nov 7, 2019 at 12:53 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > > > We use refault information to determine whether the cache workingset > > > > is stable or transitioning, and dynamically adjust the inactive:active > > > > file LRU ratio so as to maximize protection from one-off cache during > > > > stable periods, and minimize IO during transitions. > > > > > > > > With cgroups and their nested LRU lists, we currently don't do this > > > > correctly. While recursive cgroup reclaim establishes a relative LRU > > > > order among the pages of all involved cgroups, refaults only affect > > > > the local LRU order in the cgroup in which they are occuring. As a > > > > result, cache transitions can take longer in a cgrouped system as the > > > > active pages of sibling cgroups aren't challenged when they should be. > > > > > > > > [ Right now, this is somewhat theoretical, because the siblings, under > > > > continued regular reclaim pressure, should eventually run out of > > > > inactive pages - and since inactive:active *size* balancing is also > > > > done on a cgroup-local level, we will challenge the active pages > > > > eventually in most cases. But the next patch will move that relative > > > > size enforcement to the reclaim root as well, and then this patch > > > > here will be necessary to propagate refault pressure to siblings. ] > > > > > > > > This patch moves refault detection to the root of reclaim. Instead of > > > > remembering the cgroup owner of an evicted page, remember the cgroup > > > > that caused the reclaim to happen. When refaults later occur, they'll > > > > correctly influence the cross-cgroup LRU order that reclaim follows. > > > > > > I spent some time thinking about the idea of calculating refault > > > distance using target_memcg's inactive_age and then activating > > > refaulted page in (possibly) another memcg and I am still having > > > trouble convincing myself that this should work correctly. However I > > > also was unable to convince myself otherwise... We use refault > > > distance to calculate the deficit in inactive LRU space and then > > > activate the refaulted page if that distance is less that > > > active+inactive LRU size. However making that decision based on LRU > > > sizes of one memcg and then activating the page in another one seems > > > very counterintuitive to me. Maybe that's just me though... > > > > It's not activating in a random, unrelated memcg - it's the parental > > relationship that makes it work. > > > > If you have a cgroup tree > > > > root > > | > > A > > / \ > > B1 B2 > > > > and reclaim is driven by a limit in A, we are reclaiming the pages in > > B1 and B2 as if they were on a single LRU list A (it's approximated by > > the round-robin reclaim and has some caveats, but that's the idea). > > > > So when a page that belongs to B2 gets evicted, it gets evicted from > > virtual LRU list A. When it refaults later, we make the (in)active > > size and distance comparisons against virtual LRU list A as well. > > > > The pages on the physical LRU list B2 are not just ordered relative to > > its B2 peers, they are also ordered relative to the pages in B1. And > > that of course is necessary if we want fair competition between them > > under shared reclaim pressure from A. > > Thanks for clarification. The testcase in your description when group > B has a large inactive cache which does not get reclaimed while its > sibling group A has to drop its active cache got me under the > impression that sibling cgroups (in your reply above B1 and B2) can > cause memory pressure in each other. Maybe that's not a legit case and > B1 would not cause pressure in B2 without causing pressure in their > shared parent A? It now makes more sense to me and I want to confirm > that is the case. Yes. I'm sorry if this was misleading. They should only cause pressure onto each other by causing pressure on A; and then reclaim in A treats them as one combined pool of pages.
