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

[Yosry Ahmed <yosry@kernel.org>]

> > > 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.

> 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?

> If we want to make the tuning the same across swap & memory we should
> probably align on the file names and how we interact with them.

Yeah I think we should make the interfaces as consistent as possible,
within reason.