Re: [LSF/MM/BPF TOPIC] Overhauling hot page detection and promotion based on PTE A bit scanning

[David Rientjes <rientjes@google.com>]

On Fri, 24 Jan 2025, Raghavendra K T wrote:

> On 1/24/2025 11:23 AM, Hyeonggon Yoo wrote:
> > 
> > 
> > On 1/23/2025 7:57 PM, Raghavendra K T wrote:
> > > Bharata and I would like to propose the following topic for LSFMM.
> > > 
> > > Topic: Overhauling hot page detection and promotion based on PTE A bit
> > > scanning.
> [...]
> > > Here is the list of potential discussion points:
> > > 1. Other improvements and enhancements to PTE A bit scanning approach. Use
> > > of
> > > multiple kernel threads, throttling improvements, promotion policies,
> > > per-process
> > > opt-in via prctl, virtual vs physical address based scanning, tuning hot
> > > page
> > > detection algorithm etc.
> > 
> > Yuanchu's MGLRU periodic aging series [1] seems quite relevant here,
> > you might want to look at it. adding Yuanchu to Cc.
> 
> Thank you for pointing that.
> 

+1.  Yuanchu, do you have ideas for how MGLRU periodic aging and working 
set can play a role in this?

> > By the way, do you have any reason why you'd prefer opt-in prctl
> > over per-memcg control?
> > 
> 
> opt-in prctl came in the MM alignment discussion, and have added that.

Are you planning on sending a refresh of that patch series? :)

> per-memcg also definitely makes sense. I am not aware which is the most
> used usecase. But adding provision for both with one having more
> priority over other may be the way to go.
> 

I would suggest leveraging prctl() for this as opposed to memcg.  I think 
making this part of memcg is beyond the scope for what memcg is intended 
to do, limitation of memory resources, similar to the recent discussions 
on per-cgroup control for THP.

Additionally, the current memcg configuration of the system may also not 
be convenient for using for this purpose, especially if one process should 
be opted out in the memcg hierarchy.  Requiring users to change how their 
memcg is configured just to opt out would be rather unfortunate.

> Overall point here is to save time in unnecessary scanning.
> will be adding prctl in the upcoming version to start with.
> 

Fully agreed.

Thanks very much for proposing this topic, Raghu, I think it will be very 
useful to discuss!  Looking forward to it!