linux-kernel - Re: [External] Re: [PATCH] mm/memcontrol: Add the drop

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Date:   Tue, 22 Sep 2020 11:42:52 +0100
From:   Chris Down <chris@...isdown.name>
To:     Chunxin Zang <zangchunxin@...edance.com>
Cc:     Michal Hocko <mhocko@...e.com>, Yafang Shao <laoar.shao@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, lizefan@...wei.com,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, kafai@...com,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        andriin@...com, john.fastabend@...il.com, kpsingh@...omium.org,
        Cgroups <cgroups@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org
Subject: Re: [External] Re: [PATCH] mm/memcontrol: Add the drop_cache
 interface for cgroup v2

Chunxin Zang writes:
>On Tue, Sep 22, 2020 at 5:51 PM Chris Down <chris@...isdown.name> wrote:
>>
>> Chunxin Zang writes:
>> >My usecase is that there are two types of services in one server. They
>> >have difference
>> >priorities. Type_A has the highest priority, we need to ensure it's
>> >schedule latency、I/O
>> >latency、memory enough. Type_B has the lowest priority, we expect it
>> >will not affect
>> >Type_A when executed.
>> >So Type_A could use memory without any limit. Type_B could use memory
>> >only when the
>> >memory is absolutely sufficient. But we cannot estimate how much
>> >memory Type_B should
>> >use. Because everything is dynamic. So we can't set Type_B's memory.high.
>> >
>> >So we want to release the memory of Type_B when global memory is
>> >insufficient in order
>> >to ensure the quality of service of Type_A . In the past, we used the
>> >'force_empty' interface
>> >of cgroup v1.
>>
>> This sounds like a perfect use case for memory.low on Type_A, and it's pretty
>> much exactly what we invented it for. What's the problem with that?
>
>But we cannot estimate how much memory Type_A uses at least.

memory.low allows ballparking, you don't have to know exactly how much it uses.  
Any amount of protection biases reclaim away from that cgroup.

>For example:
>total memory: 100G
>At the beginning, Type_A was in an idle state, and it only used 10G of memory.
>The load is very low. We want to run Type_B to avoid wasting machine resources.
>When Type_B runs for a while, it used 80G of memory.
>At this time Type_A is busy, it needs more memory.

Ok, so set memory.low for Type_A close to your maximum expected value.