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Message-ID: <ZrzWnY9a7BFxw8Gc@PC2K9PVX.TheFacebook.com>
Date: Wed, 14 Aug 2024 12:09:01 -0400
From: Gregory Price <gourry@...rry.net>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, david@...hat.com,
ying.huang@...el.com, nphamcs@...il.com, nehagholkar@...a.com,
abhishekd@...a.com
Subject: Re: [PATCH 0/3] mm,TPP: Enable promotion of unmapped pagecache
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@...rry.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
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