Message-ID: <20250912133701.GA802874@cmpxchg.org>
Date: Fri, 12 Sep 2025 09:37:01 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: David Hildenbrand <david@...hat.com>
Cc: Kiryl Shutsemau <kas@...nel.org>, Nico Pache <npache@...hat.com>,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On Fri, Sep 12, 2025 at 02:25:31PM +0200, David Hildenbrand wrote:
> On 12.09.25 14:19, Kiryl Shutsemau wrote:
> > On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote:
> >> The following series provides khugepaged with the capability to collapse
> >> anonymous memory regions to mTHPs.
> >>
> >> To achieve this we generalize the khugepaged functions to no longer depend
> >> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
> >> pages that are occupied (!none/zero). After the PMD scan is done, we do
> >> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD
> >> range. The restriction on max_ptes_none is removed during the scan, to make
> >> sure we account for the whole PMD range. When no mTHP size is enabled, the
> >> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
> >> by the attempted collapse order to determine how full a mTHP must be to be
> >> eligible for the collapse to occur. If a mTHP collapse is attempted, but
> >> contains swapped out, or shared pages, we don't perform the collapse. It is
> >> now also possible to collapse to mTHPs without requiring the PMD THP size
> >> to be enabled.
> >>
> >> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
> >> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
> >> mTHP collapses to prevent collapse "creep" behavior. This prevents
> >> constantly promoting mTHPs to the next available size, which would occur
> >> because a collapse introduces more non-zero pages that would satisfy the
> >> promotion condition on subsequent scans.
> > 
> > Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
> > all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.
> > 
> 
> I am all for not adding any more ugliness on top of all the ugliness we 
> added in the past.
> 
> I will soon propose deprecating that parameter in favor of something 
> that makes a bit more sense.
> 
> In essence, we'll likely have an "eagerness" parameter that ranges from 
> 0 to 10. 10 is essentially "always collapse" and 0 "never collapse if 
> not all is populated".
> 
> In between we will have more flexibility on how to set these values.
> 
> Likely 9 will be around 50% to not even motivate the user to set 
> something that does not make sense (creep).

One observation we've had from production experiments is that the
optimal number here isn't static. If you have plenty of memory, then
even very sparse THPs are beneficial.

An extreme example: if all your THPs have 2/512 pages populated,
that's still cutting TLB pressure in half!

So in the absence of memory pressure, allocating and collapsing should
optimally be aggressive even on very sparse regions.

On the flipside, if there is memory pressure, TLB benefits are very
quickly drowned out by faults and paging events. And I mean real
memory pressure. If all that's happening is that somebody is streaming
through filesystem data, the optimal behavior is still to be greedy.

Another consideration is that if we need to break large folios, we
should start with colder ones that provide less benefit, and defer the
splitting of hotter ones as long as possible.

Maybe a good direction would be to move splitting out of the shrinker
and tie it to the (refault-aware) anon reclaim. And then instead of a
fixed population threshold, collapse on a pressure gradient that
starts with "no pressure/thrashing and at least two base pages in THP
a region" and ends with "reclaim is splitting everything, back off".

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