Re: [PATCH v2 2/4] mm: allow shared folios to be promoted to a fast tier

[Gregory Price]

On Thu, Sep 17, 2026 at 06:23:24PM +0200, Peter Zijlstra wrote:
> > Would be nice to just have the hardware report what's hot (and how hot)
> > rather than depending on a software-heuristic like deriving hotness
> > from a page fault.
> 
> Yeah, there is/was this patch-set from AMD that uses their IBS counters
> for this, but 'ab'-using the performance counters for this also has ick.
> PMU data isn't ideal either. Mostly they generate a ton of data that
> needs to be analyzed as well. Its not clear cut and easy.
> 
> I'm not sure there's been proposals for better hardware support.

In the CXL Spec there's a "CXL Hot-range Monitoring Unit" mechanism that
will report the N hottest regions of the device.

Not sure we actually need this for the CPU-tiers. LRU already does a
decent job of recording coldness.

I think bandwidth data provides the missing glue between the two.

I've been a proponent of just using bandwidth data to drive *when* to
promote, and let the devices dictate *what* to promote.

https://lore.kernel.org/all/aPJ7qe_2xznFSUMZ@gourry-fedora-PF4VCD3F/

e.g. if the top-tier looks under-utilized in terms of bandwidth, get
some chunk of hot data from below and see if it shifts the needle.
If so, repeat until numbers stabilize or the top tier reaches
saturation - otherwise chill out.

</food for thought | ramblings>

~Gregory