linux-kernel - Re: [RFC v2 PATCH 0/5] Promotion of Unmapped Page Cache Folios.

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Message-ID: <Z2g8xuCgqoqbpmtZ@gourry-fedora-PF4VCD3F>
Date: Sun, 22 Dec 2024 11:22:30 -0500
From: Gregory Price <gourry@...rry.net>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: Gregory Price <gourry@...rry.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, nehagholkar@...a.com,
	abhishekd@...a.com, kernel-team@...a.com, david@...hat.com,
	nphamcs@...il.com, akpm@...ux-foundation.org, hannes@...xchg.org,
	kbusch@...a.com
Subject: Re: [RFC v2 PATCH 0/5] Promotion of Unmapped Page Cache Folios.

On Sun, Dec 22, 2024 at 03:09:44PM +0800, Huang, Ying wrote:
> Gregory Price <gourry@...rry.net> writes:
> > That's 3-6% performance in this contrived case.
> 
> This is small too.
>

Small is relative.  3-6% performance increase across millions of servers
across a year is a non trivial speedup for such a common operation.

> > Can easily piggyback on that, just wasn't sure if overloading it was
> > an acceptable idea.
> 
> It's the recommended setup in the original PMEM promotion
> implementation.  Please check commit c959924b0dc5 ("memory tiering:
> adjust hot threshold automatically").
> 
> > Although since that promotion rate limit is also
> > per-task (as far as I know, will need to read into it a bit more) this
> > is probably fine.
> 
> It's not per-task.  Please read the code, especially
> should_numa_migrate_memory().

Oh, then this is already throttled.  We call mpol_misplaced which calls
should_numa_migrate_memory. 

There's some duplication of candidate selection logic between
promotion_candidate and should_numa_migrate_memory, but it may be
beneficial to keep it that way.  I'll have to look.

~Gregory