On Wed, Apr 30, 2025 at 2:15 PM Zi Yan <z...@nvidia.com> wrote: > > On 28 Apr 2025, at 14:29, Nico Pache wrote: > > > The new defer option for (m)THPs allows for a more conservative > > approach to (m)THPs. Document its usage in the transhuge admin-guide. > > > > Reviewed-by: Bagas Sanjaya <bagasdo...@gmail.com> > > Signed-off-by: Nico Pache <npa...@redhat.com> > > --- > > Documentation/admin-guide/mm/transhuge.rst | 31 ++++++++++++++++------ > > 1 file changed, 23 insertions(+), 8 deletions(-) > > > > diff --git a/Documentation/admin-guide/mm/transhuge.rst > > b/Documentation/admin-guide/mm/transhuge.rst > > index 5c63fe51b3ad..c50253357793 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 (m)THP > > How about the text below? It explicitly states khugepaged behavior. > > Khugepaged will then scan all mappings, even those not explicitly marked > with MADV_HUGEPAGE, for potential collapses into (m)THPs.
I agree, this reads better. I can modify it on the V6 :) > > > +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 (i.e., max_ptes_none=64). > > + > > .. _thp_sysfs: > > > > sysfs > > @@ -109,11 +119,14 @@ Global THP controls > > > > Transparent Hugepage Support for anonymous memory can be entirely disabled > > (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE > > -regions (to avoid the risk of consuming more memory resources) or enabled > > -system wide. This can be achieved per-supported-THP-size with one of:: > > +regions (to avoid the risk of consuming more memory resources), deferred to > > +khugepaged, or enabled system wide. > > + > > +This can be achieved per-supported-THP-size with one of:: > > > > echo always > > >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > > echo madvise > > >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > > + echo defer > > >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > > echo never > > >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > > > > where <size> is the hugepage size being addressed, the available sizes > > @@ -136,6 +149,7 @@ 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 > > > > @@ -286,7 +300,8 @@ 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`` > > > > ``max_ptes_swap`` specifies how many pages can be brought in from > > swap when collapsing a group of pages into a transparent huge page:: > > @@ -311,14 +326,14 @@ 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>``, > > where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and > > supported anonymous THP) and ``<state>`` is one of ``always``, > > ``madvise``, > > -``never`` or ``inherit``. > > +``defer``, ``never`` or ``inherit``. > > > > For example, the following will set 16K, 32K, 64K THP to ``always``, > > set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M > > Otherwise, LGTM. Thanks. Reviewed-by: Zi Yan <z...@nvidia.com> > > -- > Best Regards, > Yan, Zi >
