admin-guide/mm/transhuge.rst

12 that supports the automatic promotion and demotion of page sizes and
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
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
119 where <size> is the hugepage size being addressed, the available sizes
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
143 hugepage sizes have enabled="never". If enabling multiple hugepage
144 sizes, the kernel will select the most appropriate enabled size for a
190 	should be self-explanatory.
192 By default kernel tries to use huge, PMD-mappable zero page on read
201 PMD-mappable transparent hugepage::
207 "underused". A THP is underused if the number of zero-filled pages in
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
227    sizes.
300 You can change the sysfs boot time default for the top-level "enabled"
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
317 ``thp_anon=`` may be specified multiple times to configure all THP sizes as
318 required. If ``thp_anon=`` is specified at least once, any anon THP sizes
346 ``thp_shmem=`` may be specified multiple times to configure all THP sizes
348 sizes not explicitly configured on the command line are implicitly set to
362 it also supports smaller sizes just like anonymous memory, often referred
363 to as "multi-size THP" (mTHP). Huge pages of any size are commonly
366 While there is fine control over the huge page sizes to use for the internal
368 huge page sizes without any control over the exact sizes, behaving more like
372 ------------
390 Remember, that the kernel may use huge pages of all available sizes, and
396 ``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
408     Force the huge option on for all - very useful for testing;
411 ----------------------
418 '/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled'
422 for tmpfs mounts, except that the different huge page sizes can be controlled
424 per-size knob is set to 'inherit'.
426 The options 'force' and 'deny' are dropped for the individual sizes, which
433     Inherit the top-level "shmem_enabled" value. By default, PMD-sized hugepages
434     have enabled="inherit" and all other hugepage sizes have enabled="never";
450 transparent_hugepage/hugepages-<size>kB/enabled values and tmpfs mount
458 The number of PMD-sized anonymous transparent huge pages currently used by the
460 To identify what applications are using PMD-sized anonymous transparent huge
463 PMD-sized THP for historical reasons and should have been called
533 	queue. This happens when a huge page is partially unmapped and
534 	splitting it would free up some memory. Pages on split queue are
538 	is incremented when a huge page on the split queue was split
567 In /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats, There are
591 	is incremented every time a huge page is swapped in from a non-zswap
605 	is incremented every time a huge page is swapped out to a non-zswap
636         is incremented when a huge page is put onto split queue.
638         it would free up some memory. Pages on split queue are going to