Re: [RFC v2 2/5] mm: document transparent_hugepage=defer usage

[Nico Pache <npache@redhat.com>]

On Mon, Feb 17, 2025 at 8:04 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 11/02/2025 00:40, Nico Pache wrote:
> > The new transparent_hugepage=defer option allows for a more conservative
> > approach to THPs. Document its usage in the transhuge admin-guide.
> >
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  Documentation/admin-guide/mm/transhuge.rst | 22 +++++++++++++++++-----
> >  1 file changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> > index dff8d5985f0f..b3b18573bbb4 100644
> > --- a/Documentation/admin-guide/mm/transhuge.rst
> > +++ b/Documentation/admin-guide/mm/transhuge.rst
> > @@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wide, application
> >  may end up allocating more memory resources. An application may mmap a
> >  large region but only touch 1 byte of it, in that case a 2M page might
> >  be allocated instead of a 4k page for no good. This is why it's
> > -possible to disable hugepages system-wide and to only have them inside
> > -MADV_HUGEPAGE madvise regions.
> > +possible to disable hugepages system-wide, only have them inside
> > +MADV_HUGEPAGE madvise regions, or defer them away from the page fault
> > +handler to khugepaged.
> >
> >  Embedded systems should enable hugepages only inside madvise regions
> >  to eliminate any risk of wasting any precious byte of memory and to
> > @@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepages and that don't
> >  risk to lose memory by using hugepages, should use
> >  madvise(MADV_HUGEPAGE) on their critical mmapped regions.
> >
> > +Applications that would like to benefit from THPs but would still like a
> > +more memory conservative approach can choose 'defer'. This avoids
> > +inserting THPs at the page fault handler unless they are MADV_HUGEPAGE.
> > +Khugepaged will then scan the mappings for potential collapses into PMD
> > +sized pages. Admins using this the 'defer' setting should consider
> > +tweaking khugepaged/max_ptes_none. The current default of 511 may
> > +aggressively collapse your PTEs into PMDs. Lower this value to conserve
> > +more memory (ie. max_ptes_none=64).
> > +
>
> maybe remove the "(ie. max_ptes_none=64)", its appearing as a recommendation for
> the value, but it might not be optimal for different workloads.
>
> >  .. _thp_sysfs:
> >
> >  sysfs
> > @@ -136,6 +146,7 @@ The top-level setting (for use with "inherit") can be set by issuing
> >  one of the following commands::
> >
> >       echo always >/sys/kernel/mm/transparent_hugepage/enabled
> > +     echo defer >/sys/kernel/mm/transparent_hugepage/enabled
> >       echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
> >       echo never >/sys/kernel/mm/transparent_hugepage/enabled
> >
> > @@ -274,7 +285,8 @@ of small pages into one large page::
> >  A higher value leads to use additional memory for programs.
> >  A lower value leads to gain less thp performance. Value of
> >  max_ptes_none can waste cpu time very little, you can
> > -ignore it.
> > +ignore it. Consider lowering this value when using
> > +``transparent_hugepage=defer``
>
> lowering this value even with thp=always makes sense, as there might be cases
> when pf might not give a THP, but a VMA becomes eligable to scan via khugepaged
> later? I would remove this line.

Perhaps I should be more clear or create a different section for it.
The point was that defer was created to prevent internal fragmentation
and leave khugepaged to determine when a THP was "useful" (less
wasteful). But to achieve this less waste we should also not be using
the default.

Ideally I would want to change "always" to ignore max_ptes_none (acts
as max_ptes_none=511), and change the max_ptes_none default to 64 or
128. But that's a separate discussion that I didn't want detracting
from these postings.

>
> >
> >  ``max_ptes_swap`` specifies how many pages can be brought in from
> >  swap when collapsing a group of pages into a transparent huge page::
> > @@ -299,8 +311,8 @@ Boot parameters
> >
> >  You can change the sysfs boot time default for the top-level "enabled"
> >  control by passing the parameter ``transparent_hugepage=always`` or
> > -``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the
> > -kernel command line.
> > +``transparent_hugepage=madvise`` or ``transparent_hugepage=defer`` or
> > +``transparent_hugepage=never`` to the kernel command line.
> >
> >  Alternatively, each supported anonymous THP size can be controlled by
> >  passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``,
>