linux-kernel - Re: [PATCH 2/3] mm/vmscan: Enable kswapd to reclaim low-protected memory

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Date:   Tue, 4 Dec 2018 08:25:08 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] mm/vmscan: Enable kswapd to reclaim low-protected
 memory

On Tue 04-12-18 10:40:29, Xunlei Pang wrote:
> On 2018/12/4 AM 1:22, Michal Hocko wrote:
> > On Mon 03-12-18 23:20:31, Xunlei Pang wrote:
> >> On 2018/12/3 下午7:56, Michal Hocko wrote:
> >>> On Mon 03-12-18 16:01:18, Xunlei Pang wrote:
> >>>> There may be cgroup memory overcommitment, it will become
> >>>> even common in the future.
> >>>>
> >>>> Let's enable kswapd to reclaim low-protected memory in case
> >>>> of memory pressure, to mitigate the global direct reclaim
> >>>> pressures which could cause jitters to the response time of
> >>>> lantency-sensitive groups.
> >>>
> >>> Please be more descriptive about the problem you are trying to handle
> >>> here. I haven't actually read the patch but let me emphasise that the
> >>> low limit protection is important isolation tool. And allowing kswapd to
> >>> reclaim protected memcgs is going to break the semantic as it has been
> >>> introduced and designed.
> >>
> >> We have two types of memcgs: online groups(important business)
> >> and offline groups(unimportant business). Online groups are
> >> all configured with MAX low protection, while offline groups
> >> are not at all protected(with default 0 low).
> >>
> >> When offline groups are overcommitted, the global memory pressure
> >> suffers. This will cause the memory allocations from online groups
> >> constantly go to the slow global direct reclaim in order to reclaim
> >> online's page caches, as kswap is not able to reclaim low-protection
> >> memory. low is not hard limit, it's reasonable to be reclaimed by
> >> kswapd if there's no other reclaimable memory.
> > 
> > I am sorry I still do not follow. What role do offline cgroups play.
> > Those are certainly not low mem protected because mem_cgroup_css_offline
> > will reset them to 0.
> > 
> 
> Oh, I meant "offline groups" to be "offline-business groups", memcgs
> refered to here are all "online state" from kernel's perspective.

What is offline-business group? Please try to explain the actual problem
in much more details and do not let us guess.

-- 
Michal Hocko
SUSE Labs