On Mon, Feb 17, 2025 at 8:04 AM Usama Arif usamaarif642@gmail.com wrote:
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@redhat.com
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.
Perhaps I should be more clear or create a different section for it. The point was that defer was created to prevent internal fragmentation and leave khugepaged to determine when a THP was "useful" (less wasteful). But to achieve this less waste we should also not be using the default.
Ideally I would want to change "always" to ignore max_ptes_none (acts as max_ptes_none=511), and change the max_ptes_none default to 64 or 128. But that's a separate discussion that I didn't want detracting from these postings.
``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>``,