linux-kernel - Re: [External] Re: [PATCH V2] mm: vmscan: skip the file folios in proactive reclaim if swappiness is MAX

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Message-ID: <CACSyD1NMQp83-38FO-pHXStF=p=R82DqQwNfdx3SBQ-qNuyYUA@mail.gmail.com>
Date: Fri, 14 Mar 2025 17:24:47 +0800
From: Zhongkun He <hezhongkun.hzk@...edance.com>
To: Michal Hocko <mhocko@...e.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, muchun.song@...ux.dev, 
	yosry.ahmed@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH V2] mm: vmscan: skip the file folios in
 proactive reclaim if swappiness is MAX

On Fri, Mar 14, 2025 at 4:53 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Fri 14-03-25 11:33:50, 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.
>
> Haven't you said you will try a slightly different approach and always
> bypass LRU balancing heuristics for pro-active reclaim and swappiness
> provided? What has happened with that?
>

Hi Michal
I'm not sure if we should do that. Because i found a problem that If we
drop all the heuristics for scanning LRUs, the swappiness value each
time will accurately represent the ratio of memory to be reclaimed. This
means that before each pro reclamation operation, we would need to
have relatively clear information of the current memory ratio and dynamically
changing the swappiness more often because with the pro memory reclaiming,
the ratio of anon and file is alway changing . Therefore, we should adjust the
swappiness value more frequently.  The frequency of setting Swappiness to
200 is relatively much lower.

Do you have any commits about this concern?

> --
> Michal Hocko
> SUSE Labs