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

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Message-ID: <CAJD7tkbnGykSriLVKeZOoWxHzM6o6AfCASONFCdaeNg2oaa1LA@mail.gmail.com>
Date:   Wed, 8 Nov 2023 01:00:31 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Huan Yang <link@...o.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Hocko <mhocko@...nel.org>,
        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>,
        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

On Wed, Nov 8, 2023 at 12:22 AM Huan Yang <link@...o.com> wrote:
>
>
> 在 2023/11/8 16:14, Yosry Ahmed 写道:
> > On Wed, Nov 8, 2023 at 12:11 AM Huang, Ying <ying.huang@...el.com> wrote:
> >> Huan Yang <link@...o.com> writes:
> >>
> >>> HI Huang, Ying
> >>>
> >>> Thanks for reply.
> >>>
> >>> 在 2023/11/8 15:35, Huang, Ying 写道:
> >>>> Huan Yang <link@...o.com> writes:
> >>>>
> >>>>> 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.
> >>>>>
> >>>>> 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.
> >>>>   From mem_cgroup_swappiness(), cgroupv2 doesn't have per-cgroup
> >>>> swappiness.  So you need to add that firstly?
> >>> Sorry for this mistake, we always work on cgroupv1, so, not notice
> >>> this commit 4550c4e, thank your for point that.
> >>>
> >>> I see this commit comment that `that's a different discussion`, but,
> >>> to implements this, I will try add.
> >>>
> >>>>> 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.
> >>>> If cgroup swappiness will be adjusted temporarily, why not just change
> >>>> it via a script before/after proactive reclaiming?
> >>> IMO, this unbalance reclaim only takes effect for a single command,
> >>> so if it is pre-set using a script, the judgment of the reclamation tendency
> >>> may become complicated.
> >> If swappiness == 0, then we will only reclaim file pages.  If swappiness
> >> == 200, then we may still reclaim file pages.  So you need a way to
> >> reclaim only anon pages?
> >>
> >> If so, can we use some special swappiness value to specify that?  I
> >> don't know whether use 200 will cause regression.  If so, we may need
> >> some other value, e.g. >= 65536.
> > I don't think swappiness is the answer here. This has been discussed a
> > while back, please see my response. As you mentioned, swappiness may
> > be ignored by the kernel in some cases, and its behavior has
> > historically changed before.
>
> For type base, reclaim can have direct tendencies as well. It's good.
> But, what if
> we only want to make small adjustments to the reclamation ratio?
> Of course, sometimes swappiness may become ineffective.
>

Is there a real use case for this? I think it's difficult to reason
about swappiness and make small adjustments to the file/anon ratio
based on it. I'd prefer a more concrete implementation.