Re: [PATCH] mm: Add AnonZero accounting for zero-filled anonymous pages

[David Hildenbrand (Arm)]

On 2/16/26 18:10, Michal Hocko wrote:
> On Mon 16-02-26 17:56:12, David Hildenbrand wrote:
> > On 2/16/26 17:42, Michal Hocko wrote:
> > > You are comparing content of all anon pages, aren't you? Depending on
> > > the content this can add a lot of overhead.
> > >
> > >
> > > MADV_GET_ZEROPAGE would make more sense to me. But a more fundamental
> > > question is whether this metric is really what you want long term.
> > > Kernel can do all sorts of optimizations behind userspace back - e.g.
> > > map shared zero page - so just learning that a process has a lot of
> > > pages filled up with zeroes doesn't tell you all that much. Or does it?
> >
> > If a sysadmin wants to see where THP hurt (zero-filled pages), surely
> > MADV_GET_ZEROPAGE is the wrong (ugly) interface.
>
> The question is whether sysadmin should really ask questions like that.
> Without a deeper understanding of the workload the answer could be
> misleading at best, no matter what interface is available.

Given the requests for per-process control of THPs I assume some 
sysadmins (at hyperscalers :) ) really care about the relevant processes 
and even the relevant memory areas (e.g., jmalloc area).

> If you know and understand the workload you already know that THP is not
> a good fit and you do not need to ask about that. If you want to
> understand whether your particular workload has a big internal
> fragmentation due to THPs then MADV_GET_ZEROPAGE sounds like a
> reasonable fit to me.

You could also just scan the pages yourself for 0-content, or what's the 
benefit of letting the kernel do that?

>  From a sysadmin POV you care about the overall memory consuption, right?

I'm not so sure about that.

--
Cheers,

David