Re: [PATCH 2/3] mm: vmscan: detect file thrashing at the reclaim root

[Johannes Weiner <hannes@cmpxchg.org>]

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@cmpxchg.org> 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@cmpxchg.org> 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.