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Date:   Mon, 3 Dec 2018 18:22:02 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Xunlei Pang <xlpang@linux.alibaba.com>
Cc:     Roman Gushchin <guro@fb.com>, Johannes Weiner <hannes@cmpxchg.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/3] mm/vmscan: Enable kswapd to reclaim low-protected
 memory
Message-ID: <20181203172007.GG31738@dhcp22.suse.cz>
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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.

-- 
Michal Hocko
SUSE Labs