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

[Yosry Ahmed]

On Wed, Nov 8, 2023 at 12:11 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Huan Yang <link@xxxxxxxx> writes:
>
> > HI Huang, Ying
> >
> > Thanks for reply.
> >
> > 在 2023/11/8 15:35, Huang, Ying 写道:
> >> Huan Yang <link@xxxxxxxx> 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.