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

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Message-ID: <ZsNhgU-TiTz2WKg5@PC2K9PVX.TheFacebook.com>
Date: Mon, 19 Aug 2024 11:15:13 -0400
From: Gregory Price <gourry@...rry.net>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, david@...hat.com, nphamcs@...il.com,
	nehagholkar@...a.com, abhishekd@...a.com,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 0/3] mm,TPP: Enable promotion of unmapped pagecache

On Mon, Aug 19, 2024 at 03:46:00PM +0800, Huang, Ying wrote:
> Gregory Price <gourry@...rry.net> writes:
> 
> > 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.
> >
> > 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.
> 
> It appears that you will promote page cache page on the second access.
> Do you have some better way to identify hot pages from the not-so-hot
> pages?  How to balance between unmapped and mapped pages?  We have hot
> page selection for hot pages.
> 
> [snip]
> 

I've since explored moving this down under a (referenced && active) check.

This would be more like promotion on third access within an LRU shrink
round (the LRU should, in theory, hack off the active bits on some decent
time interval when the system is pressured).

Barring adding new counters to folios to track hits, I don't see a clear
and obvious way way to track hotness.  The primary observation here is 
that pagecache is un-mapped, and so cannot use numa-fault hints.

This is more complicated with MGLRU, but I'm saving that for after I
figure out the plan for plain old LRU.

~Gregory