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

[Yu Zhao]

On Thu, Dec 14, 2023 at 10:11 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Mon, Dec 11, 2023 at 02:55:43PM -0800, Minchan Kim wrote:
> > On Fri, Dec 08, 2023 at 10:42:29PM -0500, Johannes Weiner wrote:
> > > On Fri, Dec 08, 2023 at 03:55:59PM -0800, Chris Li wrote:
> > > > I can give you three usage cases right now:
> > > > 1) Google producting kernel uses SSD only swap, it is currently on
> > > > pilot. This is not expressible by the memory.zswap.writeback. You can
> > > > set the memory.zswap.max = 0 and memory.zswap.writeback = 1, then SSD
> > > > backed swapfile. But the whole thing feels very clunky, especially
> > > > what you really want is SSD only swap, you need to do all this zswap
> > > > config dance. Google has an internal memory.swapfile feature
> > > > implemented per cgroup swap file type by "zswap only", "real swap file
> > > > only", "both", "none" (the exact keyword might be different). running
> > > > in the production for almost 10 years. The need for more than zswap
> > > > type of per cgroup control is really there.
> > >
> > > We use regular swap on SSD without zswap just fine. Of course it's
> > > expressible.
> > >
> > > On dedicated systems, zswap is disabled in sysfs. On shared hosts
> > > where it's determined based on which workload is scheduled, zswap is
> > > generally enabled through sysfs, and individual cgroup access is
> > > controlled via memory.zswap.max - which is what this knob is for.
> > >
> > > This is analogous to enabling swap globally, and then opting
> > > individual cgroups in and out with memory.swap.max.
> > >
> > > So this usecase is very much already supported, and it's expressed in
> > > a way that's pretty natural for how cgroups express access and lack of
> > > access to certain resources.
> > >
> > > I don't see how memory.swap.type or memory.swap.tiers would improve
> > > this in any way. On the contrary, it would overlap and conflict with
> > > existing controls to manage swap and zswap on a per-cgroup basis.
> > >
> > > > 2) As indicated by this discussion, Tencent has a usage case for SSD
> > > > and hard disk swap as overflow.
> > > > https://lore.kernel.org/linux-mm/20231119194740.94101-9-ryncsn@xxxxxxxxx/
> > > > +Kairui
> > >
> > > Multiple swap devices for round robin or with different priorities
> > > aren't new, they have been supported for a very, very long time. So
> > > far nobody has proposed to control the exact behavior on a per-cgroup
> > > basis, and I didn't see anybody in this thread asking for it either.
> > >
> > > So I don't see how this counts as an obvious and automatic usecase for
> > > memory.swap.tiers.
> > >
> > > > 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.
> >
> > 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.

Someone posted it about 10 years ago :)
https://lwn.net/Articles/592923/

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Fabian recently asked me about its status.