Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user_pages loop

From: Michal Hocko <mhocko@kernel.org>
To: Yang Shi <shy828301@gmail.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	Linux MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	cgroups@vger.kernel.org,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user_pages loop
Date: Mon, 5 Aug 2019 16:32:39 +0200	[thread overview]
Message-ID: <20190805143239.GS7597@dhcp22.suse.cz> (raw)
In-Reply-To: <CAHbLzkrjh7KEvdfXackaVy8oW5CU=UaBucERffxcUorgq1vdoA@mail.gmail.com>

On Fri 02-08-19 11:56:28, Yang Shi wrote:
> On Fri, Aug 2, 2019 at 2:35 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Thu 01-08-19 14:00:51, Yang Shi wrote:
> > > On Mon, Jul 29, 2019 at 11:48 AM Michal Hocko <mhocko@kernel.org> wrote:
> > > >
> > > > On Mon 29-07-19 10:28:43, Yang Shi wrote:
> > > > [...]
> > > > > I don't worry too much about scale since the scale issue is not unique
> > > > > to background reclaim, direct reclaim may run into the same problem.
> > > >
> > > > Just to clarify. By scaling problem I mean 1:1 kswapd thread to memcg.
> > > > You can have thousands of memcgs and I do not think we really do want
> > > > to create one kswapd for each. Once we have a kswapd thread pool then we
> > > > get into a tricky land where a determinism/fairness would be non trivial
> > > > to achieve. Direct reclaim, on the other hand is bound by the workload
> > > > itself.
> > >
> > > Yes, I agree thread pool would introduce more latency than dedicated
> > > kswapd thread. But, it looks not that bad in our test. When memory
> > > allocation is fast, even though dedicated kswapd thread can't catch
> > > up. So, such background reclaim is best effort, not guaranteed.
> > >
> > > I don't quite get what you mean about fairness. Do you mean they may
> > > spend excessive cpu time then cause other processes starvation? I
> > > think this could be mitigated by properly organizing and setting
> > > groups. But, I agree this is tricky.
> >
> > No, I meant that the cost of reclaiming a unit of charges (e.g.
> > SWAP_CLUSTER_MAX) is not constant and depends on the state of the memory
> > on LRUs. Therefore any thread pool mechanism would lead to unfair
> > reclaim and non-deterministic behavior.
> 
> Yes, the cost depends on the state of pages, but I still don't quite
> understand what does "unfair" refer to in this context. Do you mean
> some cgroups may reclaim much more than others?

> Or the work may take too long so it can't not serve other cgroups in time?

exactly.
-- 
Michal Hocko
SUSE Labs