Re: [PATCH v6] zswap: memcontrol: implement zswap writeback disabling

[Christopher Li <chrisl@kernel.org>]

On Thu, Dec 14, 2023 at 9:11 AM Johannes Weiner <hannes@cmpxchg.org> wrote:

> > Hi Johannes,
> >
> > I haven't been following the thread closely, but I noticed the discussion
> > about potential use cases for zram with memcg.
> >
> > One interesting idea I have is to implement a swap controller per cgroup.
> > This would allow us to tailor the zram swap behavior to the specific needs of
> > different groups.
> >
> > For example, Group A, which is sensitive to swap latency, could use zram swap
> > with a fast compression setting, even if it sacrifices some compression ratio.
> > This would prioritize quick access to swapped data, even if it takes up more space.
> >
> > On the other hand, Group B, which can tolerate higher swap latency, could benefit
> > from a slower compression setting that achieves a higher compression ratio.
> > This would maximize memory efficiency at the cost of slightly slower data access.
> >
> > This approach could provide a more nuanced and flexible way to manage swap usage
> > within different cgroups.
>
> That makes sense to me.
>
> It sounds to me like per-cgroup swapfiles would be the easiest
> solution to this. Then you can create zram devices with different
> configurations and assign them to individual cgroups.

Ideally you need zram then following swap file after the zram. That
would be a list of the swap files rather than just one swapfile per
cgroup.

>
> This would also apply to Kairu's usecase: assign zrams and hdd backups
> as needed on a per-cgroup basis.

Same there, Kairui's request involves ZRAM and at least one extra swap
file. In other words, you really need a per cgroup swap file list.

>
> In addition, it would naturally solve scalability and isolation
> problems when multiple containers would otherwise be hammering on the
> same swap backends and locks.
>
> It would also only require one, relatively simple new interface, such
> as a cgroup parameter to swapon().
>
> That's highly preferable over a complex configuration file like
> memory.swap.tiers that needs to solve all sorts of visibility and
> namespace issues and duplicate the full configuration interface of
> every backend in some new, custom syntax.

If you don't like the syntax of memory.swap.tiers, I am open to
suggestions of your preferred syntax as well. The essicents of the
swap.tiers is a per cgroup list of the swap back ends. The names imply
that. I am not married to any given syntax of how to specify the list.
Its goal matches the above requirement pretty well.

Chris