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
55 factor of e.g. 4, 8, 16, etc), but latency spikes are much less
56 prominent because the size of each page isn't as huge as the PMD-sized
57 variant and there is less memory to clear in each page fault. Some
60 and approporiately aligned. In this case, TLB misses will occur less
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
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
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
275 A lower value leads to gain less thp performance. Value of
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
330 allocation policy for the internal shmem mount by using the kernel parameter
336 hugepage allocation policy for the tmpfs mount by using the kernel parameter
363 to as "multi-size THP" (mTHP). Huge pages of any size are commonly
372 ------------
394 using the kernel parameter ``transparent_hugepage_tmpfs=<policy>``.
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'
424 per-size knob is set to 'inherit'.
433     Inherit the top-level "shmem_enabled" value. By default, PMD-sized hugepages
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
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
687 hugetlbfs other than there will be less overall fragmentation. All