linux-kernel - Re: [RFC 0/4] Introduce unbalance proactive reclaim

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Date:   Thu, 09 Nov 2023 11:15:52 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Huan Yang <link@...o.com>
Cc:     Michal Hocko <mhocko@...e.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        "Roman Gushchin" <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Matthew Wilcox" <willy@...radead.org>,
        "Kefeng Wang" <wangkefeng.wang@...wei.com>,
        Peter Xu <peterx@...hat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Hugh Dickins <hughd@...gle.com>, <cgroups@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <opensource.kernel@...o.com>
Subject: Re: [RFC 0/4] Introduce unbalance proactive reclaim

Huan Yang <link@...o.com> writes:

> 在 2023/11/8 22:06, Michal Hocko 写道:
>> [Some people who received this message don't often get email from mhocko@...e.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>
>> On Wed 08-11-23 14:58:11, Huan Yang wrote:
>>> In some cases, we need to selectively reclaim file pages or anonymous
>>> pages in an unbalanced manner.
>>>
>>> For example, when an application is pushed to the background and frozen,
>>> it may not be opened for a long time, and we can safely reclaim the
>>> application's anonymous pages, but we do not want to touch the file pages.
>> Could you explain why? And also why do you need to swap out in that
>> case?
> When an application is frozen, it usually means that we predict that
> it will not be
> used for a long time. In order to proactively save some memory, our
> strategy will
> choose to compress the application's private data into zram. And we
> will also
> select some of the cold application data that we think is in zram and
> swap it out.
>
> The above operations assume that anonymous pages are private to the
> application.

If so, is it better only to reclaim private anonymous pages explicitly?
Add another option for that?

> After the application is frozen, compressing these pages into zram can
> save memory
> to some extent without worrying about frequent refaults.
>
> And the cost of refaults on zram is lower than that of IO.

If so, swappiness should be high system-wise?

--
Best Regards,
Huang, Ying

>
>>
>>> This patchset extends the proactive reclaim interface to achieve
>>> unbalanced reclamation. Users can control the reclamation tendency by
>>> inputting swappiness under the original interface. Specifically, users
>>> can input special values to extremely reclaim specific pages.
>> Other have already touched on this in other replies but v2 doesn't have
>> a per-memcg swappiness
>>
>>> Example:
>>>        echo "1G" 200 > memory.reclaim (only reclaim anon)
>>>          echo "1G" 0  > memory.reclaim (only reclaim file)
>>>          echo "1G" 1  > memory.reclaim (only reclaim file)
>>>
>>> Note that when performing unbalanced reclamation, the cgroup swappiness
>>> will be temporarily adjusted dynamically to the input value. Therefore,
>>> if the cgroup swappiness is further modified during runtime, there may
>>> be some errors.
>> In general this is a bad semantic. The operation shouldn't have side
>> effect that are potentially visible for another operation.
> So, maybe pass swappiness into sc and keep a single reclamation ensure that
> swappiness is not changed?
> Or, it's a bad idea that use swappiness to control unbalance reclaim.
>> --
>> Michal Hocko
>> SUSE Labs