Re: [RFC PATCH] mm/damon/reclaim: add 'available' memory as an optional watermarks metric

[Liew Rui Yan <aethernet65535@gmail.com>]

Hi SeongJae,

On Fri, 26 Jun 2026 07:18:18 -0700 SeongJae Park <sj@kernel.org> wrote:

> How about using DAMOS quota with its aim-based auto-tuning, instead?
> 
> I developed DAMOS watermarks and quota as safe guards of DAMOS.  In more
> detail, DAMOS quota is for restricting only resource usage of DAMOS.
> Meanwhile, watermarks is for restricting resource usage of both DAMON and
> DAMOS.  That is, when the watermarks condition is met, not only DAMOS but also
> DAMON is completely paused.  Also, watermarks feature was considered smarter
> than quota, because it is a kind of auto-tuning.
> 
> However, it turned out completely turning DAMON off is not a very good idea,
> because it drops monitoring results when the watermarks condition is met and
> therefore completely stops DAMON and DAMOS.  It drops the region shapes and
> 'age' information.  Especially, 'age' of some regions commonly cumulated up to
> hours or even days.  Dropping that was a problem in production use cases.

I looked at the code and noticed that kdamond_wait_activation()
doesn't seem to free the regions, the data remains in memory. By "drops",
do you mean the age information becomes stale/unreliable during the
inactive period rather than being literally discarded?

> 
> Meanwhile, we introduced aim-oriented DAMOS quota auto-tuning (a.k.a DAMOS
> quota goal).  It made DAMOS quota smarter and easier to extend than watermarks.
> Nowadays, users can also pause and resume [1] DAMON/DAMOS without losing the
> monitoring information when they want.
> 
> So, I personally don't encourage people to use watermarks.  Of course, I may
> missing some problems in alternatives.  Hence I'm asking the question to you.
> 
> How about using DAMOS quota with its aim-based auto-tuning, instead?  If you
> cannot, could you share more details?
> 
> I'll hold reviewing the code until this high level discussion is resolved.
> 
> [1] https://lore.kernel.org/20260427151231.113429-1-sj@kernel.org

Thank you for the detailed explanation. I now understand that watermarks
pause the entire DAMON monitoring loop rather than just gating DAMOS
actions. I agree this makes watermarks unsuitable for the Android use
case I described.

I will drop this patch. I think controlling DAMOS pause/rusume
separately might be better suited for execution in user space (such as
controlling‘quota_ms’ and ‘quota_sz’).

By the way, I sent you an email about three weeks ago (Fri, 5 Jun 2026
20:28:00 +0800) introducing a userspace auto-tuner program I wrote for
DAMON_RECLAIM's 'min_age', and asking for your thoughts. I am not sure
if it reached you, could you please check? I would really appreciate
your feedback on that as well.

Best regards,
Rui Yan