Re: [PATCH 0/3] mm,TPP: Enable promotion of unmapped pagecache

[Gregory Price <gourry@gourry.net>]

On Tue, Aug 13, 2024 at 11:04:59AM -0400, Gregory Price wrote:
> On Thu, Aug 08, 2024 at 04:20:11PM -0700, Andrew Morton wrote:
> > On Sat,  3 Aug 2024 05:47:12 -0400 Gregory Price <gourry@gourry.net> wrote:
> > 
> > > Unmapped pagecache pages can be demoted to low-tier memory, but 
> > > they can only be promoted if a process maps the pages into the
> > > memory space (so that NUMA hint faults can be caught).  This can
> > > cause significant performance degradation as the pagecache ages
> > > and unmapped, cached files are accessed.
> > 
> > It would be helpful to share some testing results so the magnitude of
> > this degradation can be understood.
> 
> Apologies, this should have been an RFC - testing results forthcoming.
> 
> > 
> > What is the potential downside to this change?  The local node now gets
> > stuffed full of pagecache and other things get evicted?
> > 
> 
> That is one possible degenerate case if there exists a large amount of
> free memory in the local node.  We're testing it now against TPP demotion
> logic, but the expectation should be that if the local node is already
> pressured the pagecache would be trapped on CXL until TPP frees up local
> node pages.
> 
> > > This patch series enables the pagecache to request a promotion of
> > > a folio when it is accessed via the pagecache.
> > > 
> > > We add a new `numa_hint_page_cache` counter in vmstat to capture
> > > information on when these migrations occur.

Worth noting for interested parties: This patch is not stable.  After some
extended testing, we find some soft locks.  So please disregard until v2+.

~Gregory