admin-guide/mm/transhuge.rst

26 requiring larger clear-page copy-page in page faults which is a
48 Modern kernels support "multi-size THP" (mTHP), which introduces the
50 but smaller than traditional PMD-size (as described above), in
51 increments of a power-of-2 number of pages. mTHP can back anonymous
53 PTE-mapped, but in many cases can still provide similar benefits to
56 prominent because the size of each page isn't as huge as the PMD-sized
66 collapses sequences of basic pages into PMD-sized huge pages.
91 possible to disable hugepages system-wide and to only have them inside
94 Embedded systems should enable hugepages only inside madvise regions
108 -------------------
113 system wide. This can be achieved per-supported-THP-size with one of::
115 	echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
116 	echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
117 	echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
124 	echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
127 will inherit the top-level "enabled" value::
129 	echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
133 	echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
135 The top-level setting (for use with "inherit") can be set by issuing
142 By default, PMD-sized hugepages have enabled="inherit" and all other
190 	should be self-explanatory.
192 By default kernel tries to use huge, PMD-mappable zero page on read
194 page by writing 0 or enable it back by writing 1::
201 PMD-mappable transparent hugepage::
207 "underused". A THP is underused if the number of zero-filled pages in
209 this behaviour by writing 0 to shrink_underused, and enable it by writing
215 khugepaged will be automatically started when PMD-sized THP is enabled
216 (either of the per-size anon control or the top-level control are set
218 PMD-sized THP is disabled (when both the per-size anon control and the
219 top-level control are "never")
222 -------------------
226    PMD-sized THP and no attempt is made to collapse to other THP
232 also possible to disable defrag in khugepaged by writing 0 or enable
238 You can also control how many pages khugepaged should scan at each
300 You can change the sysfs boot time default for the top-level "enabled"
301 control by passing the parameter ``transparent_hugepage=always`` or
306 passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``,
315 	thp_anon=16K-64K:always;128K,512K:inherit;256K:madvise;1M-2M:never
332 You can control hugepage allocation policy in tmpfs with mount option
350 ``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
354 There's also sysfs knob to control hugepage allocation policy for internal
366     Force the huge option on for all - very useful for testing;
368 Shmem can also use "multi-size THP" (mTHP) by adding a new sysfs knob to
369 control mTHP allocation:
370 '/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled',
380     Inherit the top-level "shmem_enabled" value. By default, PMD-sized hugepages
397 transparent_hugepage/hugepages-<size>kB/enabled values and tmpfs mount
405 The number of PMD-sized anonymous transparent huge pages currently used by the
407 To identify what applications are using PMD-sized anonymous transparent huge
410 PMD-sized THP for historical reasons and should have been called
435 	is incremented if a page fault fails to allocate or charge
439 	is incremented if a page fault fails to charge a huge page and
514 In /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats, There are
524 	is incremented if a page fault fails to allocate or charge
529 	is incremented if a page fault fails to charge a huge page and