Re: [LSF/MM/BPF TOPIC] DAMON Requirements for Access-aware MM of Future

Yuanchu Xie
2025-01-30T02:15:25Z

On Mon, Jan 13, 2025 at 7:06 PM Gregory Price <gourry@gourry.net> wrote:
> 5) Scarce resources
>
> We need to be careful not to consume excessive amounts of resources
> in an attempt to track all these identifying mechanisms. Even 1 byte
> per folio is 256MB on a 1TB machine. This gets out of hand quick.
>
> With task-work, I was able to add no additional resource consumption,
> but deferring to a fully async scenario and needing to track things
> like last-accessing CPU, timestamps, and etc.
>
> We'll need to examine this closely if we decide to aggregate either
> of these mechanisms.
My concern with physical address space monitoring is fragmentation. I
ran some numbers on a few prod machines. Grouping by regions with the
same memcg and ignoring any unmapped memory to be generous, machines
with higher utilization can have a region/total pages ratio of ~40%,
and even those with lower utilization (<50%) can also reach 20%.
Accurately tracking these regions would require quite the region
metadata, on the order of GBs.