The new defer option for (m)THPs allows for a more conservative
approach to (m)THPs. Document its usage in the transhuge admin-guide.
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 f0d4e78cedaa..d3f072bdd932 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
+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
@@ -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), defered 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
@@ -281,7 +295,8 @@ 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::
@@ -306,14 +321,14 @@ 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
--
2.48.1
