linux-kernel - Re: [RFC PATCH v2 0/8] mm: Hot page tracking and promotion infrastructure

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Message-ID: <CAEZ6=UOXG2Ju9P4-gYKnzZeyV2PAQoCA70QtmxEBJ-Dqc=0tdA@mail.gmail.com>
Date: Wed, 8 Oct 2025 10:59:01 -0700
From: Vinicius Petrucci <vpetrucci@...il.com>
To: David Rientjes <rientjes@...gle.com>
Cc: Gregory Price <gourry@...rry.net>, Matthew Wilcox <willy@...radead.org>, 
	Bharata B Rao <bharata@....com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Jonathan.Cameron@...wei.com, dave.hansen@...el.com, hannes@...xchg.org, 
	mgorman@...hsingularity.net, mingo@...hat.com, peterz@...radead.org, 
	raghavendra.kt@....com, riel@...riel.com, sj@...nel.org, weixugc@...gle.com, 
	ying.huang@...ux.alibaba.com, ziy@...dia.com, dave@...olabs.net, 
	nifan.cxl@...il.com, xuezhengchu@...wei.com, yiannis@...corp.com, 
	akpm@...ux-foundation.org, david@...hat.com, byungchul@...com, 
	kinseyho@...gle.com, joshua.hahnjy@...il.com, yuanchu@...gle.com, 
	balbirs@...dia.com, alok.rathore@...sung.com
Subject: Re: [RFC PATCH v2 0/8] mm: Hot page tracking and promotion infrastructure

Hi David,

On Tue, Sep 16, 2025 at 1:28 PM David Rientjes <rientjes@...gle.com> wrote:
>
> I've been pretty focused on the promotion story here rather than demotion
> because of how responsive it needs to be.  Harvesting the page table
> accessed bits or waiting on a sliding window through NUMA Balancing (even
> NUMAB=2) is not as responsive as needed for very fast promotion to top
> tier memory, hence things like the CHMU (or PEBS or IBS etc).

First, thanks for sharing your thoughts on the promotion
responsiveness challenges, definitely a critical aspect for tiering
strategies.

We recently put together a preliminary report using our experimental
HW that I believe could be relevant to the ongoing discussions:
A Limits Study of Memory-side Tiering Telemetry:
https://arxiv.org/abs/2508.09351

It's essentially an initial step toward quantifying the benefits of
HMU on the memory side, aiming to compare promotion quality (e.g.,
hotness coverage and accuracy) across HMU, PEBS-based promotion, and
NUMA balancing (promotion path).
Hopefully, this kind of work can help us better understand some of the
trade-offs being discussed, support more data-driven comparisons, and
spark more fruitful discussions...

Best,
Vinicius