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

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Message-ID: <Z9KPzQNctY_ALL0D@tiehlicka>
Date: Thu, 13 Mar 2025 08:57:01 +0100
From: Michal Hocko <mhocko@...e.com>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, muchun.song@...ux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V1] mm: vmscan: skip the file folios in proactive reclaim
 if swappiness is MAX

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?

-- 
Michal Hocko
SUSE Labs