Re: [External] Re: [PATCH V1] mm: vmscan: skip the file folios in proactive reclaim if swappiness is MAX

[Zhongkun He]

On Thu, Mar 13, 2025 at 3:57 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 13-03-25 11:48:12, Zhongkun He wrote:
> > With this patch 'commit <68cd9050d871> ("mm: add swappiness= arg to
> > memory.reclaim")', we can submit an additional swappiness=<val> argument
> > to memory.reclaim. It is very useful because we can dynamically adjust
> > the reclamation ratio based on the anonymous folios and file folios of
> > each cgroup. For example,when swappiness is set to 0, we only reclaim
> > from file folios.
> >
> > However,we have also encountered a new issue: when swappiness is set to
> > the MAX_SWAPPINESS, it may still only reclaim file folios. This is due
> > to the knob of cache_trim_mode, which depends solely on the ratio of
> > inactive folios, regardless of whether there are a large number of cold
> > folios in anonymous folio list.
> >
> > So, we hope to add a new control logic where proactive memory reclaim only
> > reclaims from anonymous folios when swappiness is set to MAX_SWAPPINESS.
> > For example, something like this:
> >
> > echo "2M swappiness=200" > /sys/fs/cgroup/memory.reclaim
> >
> > will perform reclaim on the rootcg with a swappiness setting of 200 (max
> > swappiness) regardless of the file folios. Users have a more comprehensive
> > view of the application's memory distribution because there are many
> > metrics available. For example, if we find that a certain cgroup has a
> > large number of inactive anon folios, we can reclaim only those and skip
> > file folios, because with the zram/zswap, the IO tradeoff that
> > cache_trim_mode is making doesn't hold - file refaults will cause IO,
> > whereas anon decompression will not.
> >
> > With this patch, the swappiness argument of memory.reclaim has a more
> > precise semantics: 0 means reclaiming only from file pages, while 200
> > means reclaiming just from anonymous pages.
>
> Well, with this patch we have 0 - always swap, 200 - never swap and
> anything inbetween behaves more or less arbitrary, right? Not a new
> problem with swappiness but would it make more sense to drop all the
> heuristics for scanning LRUs and simply use the given swappiness when
> doing the pro active reclaim?
>

Thanks for your suggestion! I totally agree with you. I'm preparing to send
another patch to do this and a new thread to discuss, because I think the
implementation doesn't conflict with this one. Do you think so ?

> --
> Michal Hocko
> SUSE Labs