[Linux Memory Hotness and Promotion] Notes from March 12, 2026

[David Rientjes <rientjes@google.com>]

Hi everybody,

Here are the notes from the last Linux Memory Hotness and Promotion call
that happened on Thursday, March 12.  Thanks to everybody who was 
involved!

These notes are intended to bring people up to speed who could not attend 
the call as well as keep the conversation going in between meetings.

----->o-----
Bharata updated that he is working on v6 of his patch series.  He noted 
significant work being done on NUMAB since in the initial work he had 
bypassed the regular balancing mode; he made the NUMA hint faults work 
only with hot page promotion.  In his next series of changes, he added 
back proper NUMAB=1 support.

This turned out to require that NUMA Balancing would start to depend on 
pghot.  All of the functions are required by pghot and this will make it 
easier when future sources of page hotness come later.

A concern was earlier raised about memcg stats reporting which should now 
be fixed.  There were other small fixes merged as well.  It will take a 
week or so before patches will be ready to post.  v6 should be posted by 
the next meeting.  Nobody expressed direct concerns about this approach.

----->o-----
We chatted about the upcoming LSF/MM/BPF conference.  Bharata's suggestion 
for his series was to continue the discussion on the mailing list since it 
was covered in last LSF/MM/BPF and then more recently at LPC.

----->o-----
Joshua had been doing some testing with tier-aware memcg limits.  He found 
in the general case when everything is behaving as appropriate that there 
weren't major benefits.  When there is a memcg performing poorly, however, 
this can punish those workloads as needed.  He showed a graph that showed 
a very wide range of performance with NUMAB=2 and then a much smaller 
variance with tier-aware memcg limits.  The important part is that since 
the variance is low that you then have direct control over where the 
average throughput could be.  He is going to be including this data in the 
v2 posting of his patch series.

He wanted to get feedback from memcg stakeholders upstream and discussed 
providing some guidance to end users on how to put these pieces of the 
puzzle together.  This includes translating tier-unaware memcg limits into 
tier-aware memcg limits.

Gregory noted that one of the next steps will be to test both tier-aware 
memcg limits with Bharata's series.  Bharata would also take a look at 
Joshua's series.

Joshua sought feedback on the best way to compare with and without 
tier-aware memcg limits.  Gregory said that we'll eventually move to 
workload testing but in the meantime it would be useful to understand if 
the floor of the throughput raises when you start to use pghot -- this 
would be an outright win.  Joshua was planning on having pghot + 
tier-aware memcg limits data to share for LSF/MM/BPF.

----->o-----
Shivank updated on the progress for hardware assists for migrate_pages().  
He said that he posted a v4 of the patch series a couple days ago that 
includes several changes.  He pivoted to doing pre-copy of data from the 
source to the destination and, if successful, then the metadata is 
updated.

There were no major blockers for this work.  David Hildenbrand suggested a 
few minor changes.

We also touched on non-temporal stores and enlightenment for 
migrate_pages().  Yiannis started work on this, there was some feedback 
upstream for a new migrate mode to be supported.  Yiannis has been 
discussing this with his colleagues and will be able to update; he is 
targeting having patches on the list in the next two weeks.

Wei Xu had a similar approach to the new migrate mode and ended up 
splitting out a helper function that would check for both MIGRATE_ASYNC as 
well as the new mode.

----->o-----
Next meeting will be on Thursday, March 26 at 8:30am PDT (UTC-7),
everybody is welcome: https://meet.google.com/jak-ytdx-hnm

Topics for the next meeting:

 - v6 of Bharata's patch series, including NUMA Balancing dependency on
   pghot
 - v2 of tier-aware memcg limits (reclaim fairness)
 - v4 of enlightening migrate_pages() for hardware assists and how this
   work will be charged to userspace, including for memory compaction
 - non-temporal stores enlightenment for memory tiering
 - Gregory's testing of tier aware memcg limits with Bharata's changes
 - discuss generalized subsystem for providing bandwidth information
   independent of the underlying platform, ideally through resctrl,
   otherwise utilizing bandwidth information will be challenging
   + preferably this bandwidth monitoring is not per NUMA node but rather
     slow and fast

Please let me know if you'd like to propose additional topics for
discussion, thank you!