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@xxxxxxxxxx> 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
