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

[Andrew Morton <akpm@linux-foundation.org>]

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.

What is the potential downside to this change?  The local node now gets
stuffed full of pagecache and other things get evicted?

> 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.