Re: [RFC PATCH] mm: Add reclaim type to memory.reclaim

[Michal Hocko <mhocko@suse.com>]

On Tue 27-02-24 13:48:31, Yafang Shao wrote:
> On Mon, Feb 26, 2024 at 10:05 PM Michal Hocko <mhocko@suse.com> wrote:
[...]
> > > To manage disk
> > > storage efficiently, we employ an agent that identifies container images
> > > eligible for destruction once all instances of that image exit.
> > >
> > > However, during destruction, dealing with directories containing numerous
> > > negative dentries can significantly impact performance.
> >
> > Performance of what. I have to say I am kind of lost here. We are
> > talking about memory or a disk storage?
> 
> Removing an empty directory with numerous dentries can significantly
> prolong the process of freeing associated dentries, leading to high
> system CPU usage that adversely affects overall system performance.

Is there anything that prevents you from reclaiming the memcg you are
about to remove? We do have interfaces for that.

> > > To mitigate this
> > > issue, we aim to proactively reclaim these dentries using a user agent.
> > > Extending the memory.reclaim functionality to specifically target slabs
> > > aligns with our requirements.
> >
> > Matthew has already pointed out that this has been proposed several
> > times already and rejected.
> 
> With that being said, we haven't come up with any superior solutions
> compared to the proposals mentioned.
> 
> > Dedicated slab shrinking interface is
> > especially tricky because you would need a way to tell which shrinkers
> > to invoke and that would be very kernel version specific.
> 
> The persistence of this issue over several years without any
> discernible progress suggests that we might be heading in the wrong
> direction. Perhaps we could consider providing a kernel interface to
> users, allowing them to tailor the reclamation process based on their
> workload requirements.

There are clear problems identified with type specific reclaim and those
might easily strike back with future changes. Once we put an interface
in place we won't be able remove it and that could lead to problems with
future changes in the memory reclaim.
-- 
Michal Hocko
SUSE Labs