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@xxxxxxxxxx> > --- > 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. > > ``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>``,
