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

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Message-ID: <Zrt2GxQioCYiZ2fg@PC2K9PVX.TheFacebook.com>
Date: Tue, 13 Aug 2024 11:04:59 -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 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.