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

[Kairui Song]

Chris Li <chrisl@xxxxxxxxxx> 于2023年12月13日周三 07:58写道：
>
> Hi Minchan,
>
> On Mon, Dec 11, 2023 at 2:55 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> > > > 3) Android has some fancy swap ideas led by those patches.
> > > > https://lore.kernel.org/linux-mm/20230710221659.2473460-1-minchan@xxxxxxxxxx/
> > > > It got shot down due to removal of frontswap. But the usage case and
> > > > product requirement is there.
> > > > +Minchan
> > >
> > > This looks like an optimization for zram to bypass the block layer and
> > > hook directly into the swap code. Correct me if I'm wrong, but this
> > > doesn't appear to have anything to do with per-cgroup backend control.
> >
> > 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.
>
> That is a very solid usage case. Thanks for sharing this swap backend
> usage story. It goes beyond my original memory.swap.teires idea as
> well.
>
> We can have some zram specific knobs to control what compression
> setting is using. Moving data between different compression settings
> would be an interesting topic. It might fit the swap.tiers usage model
> as well. I am just thinking it out loud. Maybe define different
> compression settings as different tiers and then allow the cgroup to
> enroll into one of the tiers list.
>

This is very similar to our usage, easy to implement too. Actually,
now ZRAM already supports multiple compression streams, so if each
memcg just provides an extra knob to record the compression level
(1-4), ZRAM can decide which compression stream to use when the page
reaches ZRAM, just by checking pages memcg. It's limited to 4 levels
but enough for most cases.