Re: [RFC PATCH 0/6] mm/memcontrol: Make memcg limits tier-aware

[Gregory Price <gourry@gourry.net>]

On Tue, Feb 24, 2026 at 08:13:56AM -0800, Joshua Hahn wrote:
> ... snip ...

Just injecting a few points here
(disclosure: I have been in the development loop for this feature)

> 
> > Otherwise promotions would make sure to that we have the most active
> > memory in the top tier.
> 

Yes / No.  This makes the assumption that you always want this.

Barring a minimum Quality of Service mechanism (as Joshua explains)
this reduces the usefulness of a secondary tier of memory.

Services will just prefer not to be deployed to these kinds of
machines because the performance variance is too high.

> 
> > Is this typical in real life configurations?
> 
> I would say so. I think that the two examples above are realistic
> scenarios that cloud providers and hyperscalers might face on tiered systems.
> 

The answer is unequivocally yes.

Lacking tier-awareness is actually a huge blocker for deploying mixed
workloads on large, dense memory systems with multiple tiers (2+).

Technically we're already at 4-ish tiers: DDR, CXL, ZSWAP, SWAP.

We have zswap/swap controls in cgroups already, we just lack that same
control for coherent memory tiers.  This tries to use the existing nobs
(max/high/low/min) to do what they already do - just proportionally.

~Gregory