| Message ID | 20250414220557.35388-13-npache@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | khugepaged: mTHP support | expand |
On 4/14/25 3:05 PM, Nico Pache wrote: > Now that we can collapse to mTHPs lets update the admin guide to > reflect these changes and provide proper guidence on how to utilize it. > > Signed-off-by: Nico Pache <npache@redhat.com> > --- > Documentation/admin-guide/mm/transhuge.rst | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst > index dff8d5985f0f..f0d4e78cedaa 100644 > --- a/Documentation/admin-guide/mm/transhuge.rst > +++ b/Documentation/admin-guide/mm/transhuge.rst > @@ -63,7 +63,7 @@ often. > THP can be enabled system wide or restricted to certain tasks or even > memory ranges inside task's address space. Unless THP is completely > disabled, there is ``khugepaged`` daemon that scans memory and > -collapses sequences of basic pages into PMD-sized huge pages. > +collapses sequences of basic pages into huge pages. > > The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>` > interface and using madvise(2) and prctl(2) system calls. > @@ -144,6 +144,13 @@ hugepage sizes have enabled="never". If enabling multiple hugepage > sizes, the kernel will select the most appropriate enabled size for a > given allocation. > > +khugepaged uses max_ptes_none scaled to the order of the enabled mTHP size to > +determine collapses. When using mTHPs its recommended to set max_ptes_none low. it's > +Ideally less than HPAGE_PMD_NR / 2 (255 on 4k page size). This will prevent ^^^ not a sentence > +undesired "creep" behavior that leads to continuously collapsing to a larger > +mTHP size. max_ptes_shared and max_ptes_swap have no effect when collapsing to a > +mTHP, and mTHP collapse will fail on shared or swapped out pages. > + > It's also possible to limit defrag efforts in the VM to generate > anonymous hugepages in case they're not immediately free to madvise > regions or to never try to defrag memory and simply fallback to regular
diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst index dff8d5985f0f..f0d4e78cedaa 100644 --- a/Documentation/admin-guide/mm/transhuge.rst +++ b/Documentation/admin-guide/mm/transhuge.rst @@ -63,7 +63,7 @@ often. THP can be enabled system wide or restricted to certain tasks or even memory ranges inside task's address space. Unless THP is completely disabled, there is ``khugepaged`` daemon that scans memory and -collapses sequences of basic pages into PMD-sized huge pages. +collapses sequences of basic pages into huge pages. The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>` interface and using madvise(2) and prctl(2) system calls. @@ -144,6 +144,13 @@ hugepage sizes have enabled="never". If enabling multiple hugepage sizes, the kernel will select the most appropriate enabled size for a given allocation. +khugepaged uses max_ptes_none scaled to the order of the enabled mTHP size to +determine collapses. When using mTHPs its recommended to set max_ptes_none low. +Ideally less than HPAGE_PMD_NR / 2 (255 on 4k page size). This will prevent +undesired "creep" behavior that leads to continuously collapsing to a larger +mTHP size. max_ptes_shared and max_ptes_swap have no effect when collapsing to a +mTHP, and mTHP collapse will fail on shared or swapped out pages. + It's also possible to limit defrag efforts in the VM to generate anonymous hugepages in case they're not immediately free to madvise regions or to never try to defrag memory and simply fallback to regular
Now that we can collapse to mTHPs lets update the admin guide to reflect these changes and provide proper guidence on how to utilize it. Signed-off-by: Nico Pache <npache@redhat.com> --- Documentation/admin-guide/mm/transhuge.rst | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)