@@ -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).
+
.. _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
@@ -264,7 +275,8 @@ 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::
The new transparent_hugepage=defer option allows for a more conservative approach to THPs. Document its usage in the transhuge admin-guide. Cc: Andrew Morton <akpm@linux-foundation.org> Cc: David Hildenbrand <david@redhat.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Barry Song <baohua@kernel.org> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: Baolin Wang <baolin.wang@linux.alibaba.com> Cc: Lance Yang <ioworker0@gmail.com> Cc: Peter Xu <peterx@redhat.com> Cc: Zi Yan <ziy@nvidia.com> Cc: Rafael Aquini <aquini@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Signed-off-by: Nico Pache <npache@redhat.com> --- Documentation/admin-guide/mm/transhuge.rst | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-)