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

[Michal Hocko]

On Thu 13-03-25 16:57:34, Zhongkun He wrote:
> 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 ?

If the change will enforce SCAN_FRACT for proactive reclaim with
swappiness given then it will make the balancing much smoother but I do
not think the behavior at both ends of the scale would imply only single
LRU scanning mode.
-- 
Michal Hocko
SUSE Labs