Re: [RFC PATCH v1 0/4] mm/damon: Support hot application detections

[SeongJae Park <sj@kernel.org>]

Hello Asier,


Thank you for sharing this nice RFC patch series!

On Mon, 2 Feb 2026 14:56:45 +0000 <gutierrez.asier@huawei-partners.com> wrote:

> From: Asier Gutierrez <gutierrez.asier@huawei-partners.com>
> 
> Overview
> ----------
> 
> This patch set introduces a new dynamic mechanism for detecting hot applications
> and hot regions in those applications.
> 
> Motivation
> -----------
> 
> Currently DAMON requires the system administrator to provide information about
> which application needs to be monitored and all the parameters. Ideally this
> should be done automatically, with minimal intervention from the system
> administrator.
> 
> 
> Since TLB is a bottleneck for many systems, a way to optimize TLB misses (or
> hits) is to use huge pages. Unfortunately, using "always" in THP leads to memory
> fragmentation and memory waste. For this reason, most application guides and
> system administrators suggest to disable THP.
> 
> We would like to detect: 1. which applications are hot in the system and 2.
> which memory regions are hot in order to collapse those regions.
> 
> 
> Solution
> -----------
> 
>      ┌────────────┐           ┌────────────┐
>      │Damon_module│           │Task_monitor│
>      └──────┬─────┘           └──────┬─────┘
>             │         start          │
>             │───────────────────────>│
>             │                        │
>             │                        │────┐
>             │                        │    │ calculate task load
>             │                        │<───┘
>             │                        │
>             │                        │────┐
>             │                        │    │ sort tasks
>             │                        │<───┘
>             │                        │
>             │                        │────┐
>             │                        │    │ start kdamond for top 3 tasks
>             │                        │<───┘
>      ┌──────┴─────┐           ┌──────┴─────┐
>      │Damon_module│           │Task_monitor│
>      └────────────┘           └────────────┘
> 
> 
> We calculate the task load base on the sum of all the utime for all the threads
> in a given task. Once we get total utime, we use the exponential load average
> provided by calc_load. The tasks that become cold, the kdamond will be stopped
> for them.

Sounds interesting, and this high level idea makes sense to me. :)

I'd like to further learn a few things.  Is there a reason to think the top 3
tasks are enough number of tasks?  Also, what if a region was hot and
successfully promoted to use huge pages, but later be cold?  Should we also
have a DAMOS scheme for splitting such no-more-hot huge pages?

> 
> In each kdamond, we start with a high min_access value. Our goal is to find the
> "maximum" min_access value at which point the DAMON action is applied. In each
> cycle, if no action is applied, we lower the min_access.

Sounds like a nice auto-tuning.  And we have DAMOS quota goal for that kind of
auto-tuning.  Have you considered using that?

> 
> Regarding the action, we introduce a new action: DAMOS_COLLAPSE. This allows us
> collapse synchronously and avoid polluting khugepaged and other parts of the MM
> subsystem with DAMON stuff. DAMOS_HUGEPAGE eventually calls hugepage_madvise,
> which needs the correct vm_flags_t set.
> 
> Benchmark
> -----------

Seems you forgot writing this section up.  Or, you don't have benchmark results
yet, but only mistakenly wrote the above section header?  Either is fine, as
this is just an RFC.  Nevertheless, test results and your expected use case of
this patch series will be very helpful.


Thanks,
SJ

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