On 4/14/25 3:24 PM, 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.
Signed-off-by: Nico Pache npache@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).
i.e.,
.. _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
deferred
+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