admin-guide/mm/transhuge.rst

26 requiring larger clear-page copy-page in page faults which is a
36 1) the TLB miss will run faster (especially with virtualization using
40 2) a single TLB entry will be mapping a much larger amount of virtual
41    memory in turn reducing the number of TLB misses. With
42    virtualization and nested pagetables the TLB can be mapped of
45    the two is using hugepages just because of the fact the TLB miss is
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
58 architectures also employ TLB compression mechanisms to squeeze more
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 or by setting "inherit" while the top-level enabled is set to "always"
210 setting "inherit" while the top-level enabled is set to "never").
213 -------------------
217    PMD-sized THP and no attempt is made to collapse to other THP
316 ``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
332     Force the huge option on for all - very useful for testing;
338 transparent_hugepage/hugepages-<size>kB/enabled values and tmpfs mount
348    PMD-sized THP. Events relating to other THP sizes are not included.
350 The number of PMD-sized anonymous transparent huge pages currently used by the
352 To identify what applications are using PMD-sized anonymous transparent huge
355 PMD-sized THP for historical reasons and should have been called
411 	is incremented every time a huge page is split into base
417 	is incremented if kernel fails to split huge
421 	is incremented when a huge page is put onto split
423 	splitting it would free up some memory. Pages on split queue are
424 	going to be split under memory pressure.
427 	is incremented every time a PMD split into table of PTEs.
429 	munmap() on part of huge page. It doesn't split huge page, only
446 	is incremented if a huge page has to be split before swapout.