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

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Message-ID: <ZtcQ8sgr_Wgu3pB0@PC2K9PVX.TheFacebook.com>
Date: Tue, 3 Sep 2024 09:36:50 -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>,
	Feng Tang <feng.tang@...el.com>
Subject: Re: [PATCH 0/3] mm,TPP: Enable promotion of unmapped pagecache

On Mon, Sep 02, 2024 at 02:53:26PM +0800, Huang, Ying wrote:
> Gregory Price <gourry@...rry.net> writes:
> 
> > 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.
> 
> Several years ago, we have tried to use the access time tracking
> mechanism of NUMA balancing to track the access time latency of unmapped
> file cache folios.  The original implementation is as follows,
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/commit/?h=tiering-0.8&id=5f2e64ce75c0322602c2ec8c70b64bb69b1f1329
> 
> What do you think about this?
>

Also seems like an interesting option. I've been looking at another old
proposal to simply add a new LRU that was implemented by kbusch a few
years back.

https://git.kernel.org/pub/scm/linux/kernel/git/kbusch/linux.git/commit/?h=lru-promote&id=6616afe9a722f6ebedbb27ade3848cf07b9a3af7

I may spend a little time to add a few different methods in with a switch
I can flip to test them side by side / with each other and see what results
we can get.
 
> --
> Best Regards,
> Huang, Ying