Re: [PATCH v9 3/6] mm: memcontrol: add interface for swap tier selection

[Joshua Hahn]

On Mon, 22 Jun 2026 23:46:31 +0000 Yosry Ahmed <yosry@xxxxxxxxxx> wrote:

> > > > If that is the case, I think auto-scaling makes sense but can be a bit
> > > > tricky, since there is no universal tiered ratio; each workload will
> > > > have different tiers it can swap to, so they will all have to calculate
> > > > their own ratios. Tiered memory limits escapes this difficulty since we
> > > > assume all memory can be placed on all tiers, so we have a system-wide
> > > > ratio : -)
> > > 
> > > Hmm I don't follow. It's also possible (maybe not initially) that a
> > > memcg cannot use specific memory tiers, right? I am not sure what the
> > > difference is.
> > 
> > You're right, I was speaking more to the current state of memory tiers.
> > The majority of the feedack I received was that we already have too
> > many memcg knobs, so I just opted to make tiered memcg limits a
> > cgroup mount, with no ability for individual memcgs to tune their
> > limits or opt-in/out.
> 
> Right, I think this is similar to the approach taken here. We have a
> single interface for per-tier limits. The main difference is that we're
> allowing 0/max values to disable/enable different swap tiers per-memcg,
> as there's a use case for that.
> 
> Seems like for memory tiering there's no use case for that yet.

Yes, I would agree with that.

> > What do you think Yosry? Would it make sense for us to be able to 
> > tune these values? Personally I think it makes sense but just wanted to
> > make the basic features merged before I went to push for making those
> > knobs tunable.
> 
> Right now we're not proposing to allow tuning swap tier limits either,
> just enable or disable a tier. My main question is about the default
> values.
> 
> IIUC, for memory tiering, if you set memory.max, then the limits for
> tiers are auto-scaled. I think it makes sense to do the same for swap
> tiers for cosnsitency. Or am I wrong about the memory tiering limits
> behavior?

No, you're right about that. Sorry for steering the thread to my 
series ; -)

To get back to the question of how the auto-tuning should work, the
main question is to which ratio we scale the swap limits to.
Do we set the swap limits proportional to how much swap is present
in the system, or how much swap is available to the cgroup?

So if we have 3 swap tiers A, B, C, with 50G, 30G, and 20G capacity
respectively, how much should a cgroup with swap.max = 10G have if
it is limited to tiers A and B?

This is what I was getting at earlier when I said we have to calculate
different ratios for different cgroups, based on what tiers they have
access to.

Sorry if that doesn't make sense, please let me know how I can
elaborate!