1=============
2HugeTLB Pages
3=============
4
5Overview
6========
7
8The intent of this file is to give a brief summary of hugetlbpage support in
9the Linux kernel.  This support is built on top of multiple page size support
10that is provided by most modern architectures.  For example, x86 CPUs normally
11support 4K and 2M (1G if architecturally supported) page sizes, ia64
12architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
13256M and ppc64 supports 4K and 16M.  A TLB is a cache of virtual-to-physical
14translations.  Typically this is a very scarce resource on processor.
15Operating systems try to make best use of limited number of TLB resources.
16This optimization is more critical now as bigger and bigger physical memories
17(several GBs) are more readily available.
18
19Users can use the huge page support in Linux kernel by either using the mmap
20system call or standard SYSV shared memory system calls (shmget, shmat).
21
22First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
23(present under "File systems") and CONFIG_HUGETLB_PAGE (selected
24automatically when CONFIG_HUGETLBFS is selected) configuration
25options.
26
27The ``/proc/meminfo`` file provides information about the total number of
28persistent hugetlb pages in the kernel's huge page pool.  It also displays
29default huge page size and information about the number of free, reserved
30and surplus huge pages in the pool of huge pages of default size.
31The huge page size is needed for generating the proper alignment and
32size of the arguments to system calls that map huge page regions.
33
34The output of ``cat /proc/meminfo`` will include lines like::
35
36	HugePages_Total: uuu
37	HugePages_Free:  vvv
38	HugePages_Rsvd:  www
39	HugePages_Surp:  xxx
40	Hugepagesize:    yyy kB
41	Hugetlb:         zzz kB
42
43where:
44
45HugePages_Total
46	is the size of the pool of huge pages.
47HugePages_Free
48	is the number of huge pages in the pool that are not yet
49        allocated.
50HugePages_Rsvd
51	is short for "reserved," and is the number of huge pages for
52        which a commitment to allocate from the pool has been made,
53        but no allocation has yet been made.  Reserved huge pages
54        guarantee that an application will be able to allocate a
55        huge page from the pool of huge pages at fault time.
56HugePages_Surp
57	is short for "surplus," and is the number of huge pages in
58        the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
59        maximum number of surplus huge pages is controlled by
60        ``/proc/sys/vm/nr_overcommit_hugepages``.
61	Note: When the feature of freeing unused vmemmap pages associated
62	with each hugetlb page is enabled, the number of surplus huge pages
63	may be temporarily larger than the maximum number of surplus huge
64	pages when the system is under memory pressure.
65Hugepagesize
66	is the default hugepage size (in kB).
67Hugetlb
68        is the total amount of memory (in kB), consumed by huge
69        pages of all sizes.
70        If huge pages of different sizes are in use, this number
71        will exceed HugePages_Total \* Hugepagesize. To get more
72        detailed information, please, refer to
73        ``/sys/kernel/mm/hugepages`` (described below).
74
75
76``/proc/filesystems`` should also show a filesystem of type "hugetlbfs"
77configured in the kernel.
78
79``/proc/sys/vm/nr_hugepages`` indicates the current number of "persistent" huge
80pages in the kernel's huge page pool.  "Persistent" huge pages will be
81returned to the huge page pool when freed by a task.  A user with root
82privileges can dynamically allocate more or free some persistent huge pages
83by increasing or decreasing the value of ``nr_hugepages``.
84
85Note: When the feature of freeing unused vmemmap pages associated with each
86hugetlb page is enabled, we can fail to free the huge pages triggered by
87the user when the system is under memory pressure.  Please try again later.
88
89Pages that are used as huge pages are reserved inside the kernel and cannot
90be used for other purposes.  Huge pages cannot be swapped out under
91memory pressure.
92
93Once a number of huge pages have been pre-allocated to the kernel huge page
94pool, a user with appropriate privilege can use either the mmap system call
95or shared memory system calls to use the huge pages.  See the discussion of
96:ref:`Using Huge Pages <using_huge_pages>`, below.
97
98The administrator can allocate persistent huge pages on the kernel boot
99command line by specifying the "hugepages=N" parameter, where 'N' = the
100number of huge pages requested.  This is the most reliable method of
101allocating huge pages as memory has not yet become fragmented.
102
103Some platforms support multiple huge page sizes.  To allocate huge pages
104of a specific size, one must precede the huge pages boot command parameters
105with a huge page size selection parameter "hugepagesz=<size>".  <size> must
106be specified in bytes with optional scale suffix [kKmMgG].  The default huge
107page size may be selected with the "default_hugepagesz=<size>" boot parameter.
108
109Hugetlb boot command line parameter semantics
110
111hugepagesz
112	Specify a huge page size.  Used in conjunction with hugepages
113	parameter to preallocate a number of huge pages of the specified
114	size.  Hence, hugepagesz and hugepages are typically specified in
115	pairs such as::
116
117		hugepagesz=2M hugepages=512
118
119	hugepagesz can only be specified once on the command line for a
120	specific huge page size.  Valid huge page sizes are architecture
121	dependent.
122hugepages
123	Specify the number of huge pages to preallocate.  This typically
124	follows a valid hugepagesz or default_hugepagesz parameter.  However,
125	if hugepages is the first or only hugetlb command line parameter it
126	implicitly specifies the number of huge pages of default size to
127	allocate.  If the number of huge pages of default size is implicitly
128	specified, it can not be overwritten by a hugepagesz,hugepages
129	parameter pair for the default size.  This parameter also has a
130	node format.  The node format specifies the number of huge pages
131	to allocate on specific nodes.
132
133	For example, on an architecture with 2M default huge page size::
134
135		hugepages=256 hugepagesz=2M hugepages=512
136
137	will result in 256 2M huge pages being allocated and a warning message
138	indicating that the hugepages=512 parameter is ignored.  If a hugepages
139	parameter is preceded by an invalid hugepagesz parameter, it will
140	be ignored.
141
142	Node format example::
143
144		hugepagesz=2M hugepages=0:1,1:2
145
146	It will allocate 1 2M hugepage on node0 and 2 2M hugepages on node1.
147	If the node number is invalid,  the parameter will be ignored.
148
149default_hugepagesz
150	Specify the default huge page size.  This parameter can
151	only be specified once on the command line.  default_hugepagesz can
152	optionally be followed by the hugepages parameter to preallocate a
153	specific number of huge pages of default size.  The number of default
154	sized huge pages to preallocate can also be implicitly specified as
155	mentioned in the hugepages section above.  Therefore, on an
156	architecture with 2M default huge page size::
157
158		hugepages=256
159		default_hugepagesz=2M hugepages=256
160		hugepages=256 default_hugepagesz=2M
161
162	will all result in 256 2M huge pages being allocated.  Valid default
163	huge page size is architecture dependent.
164hugetlb_free_vmemmap
165	When CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is set, this enables HugeTLB
166	Vmemmap Optimization (HVO).
167
168When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
169indicates the current number of pre-allocated huge pages of the default size.
170Thus, one can use the following command to dynamically allocate/deallocate
171default sized persistent huge pages::
172
173	echo 20 > /proc/sys/vm/nr_hugepages
174
175This command will try to adjust the number of default sized huge pages in the
176huge page pool to 20, allocating or freeing huge pages, as required.
177
178On a NUMA platform, the kernel will attempt to distribute the huge page pool
179over all the set of allowed nodes specified by the NUMA memory policy of the
180task that modifies ``nr_hugepages``. The default for the allowed nodes--when the
181task has default memory policy--is all on-line nodes with memory.  Allowed
182nodes with insufficient available, contiguous memory for a huge page will be
183silently skipped when allocating persistent huge pages.  See the
184:ref:`discussion below <mem_policy_and_hp_alloc>`
185of the interaction of task memory policy, cpusets and per node attributes
186with the allocation and freeing of persistent huge pages.
187
188The success or failure of huge page allocation depends on the amount of
189physically contiguous memory that is present in system at the time of the
190allocation attempt.  If the kernel is unable to allocate huge pages from
191some nodes in a NUMA system, it will attempt to make up the difference by
192allocating extra pages on other nodes with sufficient available contiguous
193memory, if any.
194
195System administrators may want to put this command in one of the local rc
196init files.  This will enable the kernel to allocate huge pages early in
197the boot process when the possibility of getting physical contiguous pages
198is still very high.  Administrators can verify the number of huge pages
199actually allocated by checking the sysctl or meminfo.  To check the per node
200distribution of huge pages in a NUMA system, use::
201
202	cat /sys/devices/system/node/node*/meminfo | fgrep Huge
203
204``/proc/sys/vm/nr_overcommit_hugepages`` specifies how large the pool of
205huge pages can grow, if more huge pages than ``/proc/sys/vm/nr_hugepages`` are
206requested by applications.  Writing any non-zero value into this file
207indicates that the hugetlb subsystem is allowed to try to obtain that
208number of "surplus" huge pages from the kernel's normal page pool, when the
209persistent huge page pool is exhausted. As these surplus huge pages become
210unused, they are freed back to the kernel's normal page pool.
211
212When increasing the huge page pool size via ``nr_hugepages``, any existing
213surplus pages will first be promoted to persistent huge pages.  Then, additional
214huge pages will be allocated, if necessary and if possible, to fulfill
215the new persistent huge page pool size.
216
217The administrator may shrink the pool of persistent huge pages for
218the default huge page size by setting the ``nr_hugepages`` sysctl to a
219smaller value.  The kernel will attempt to balance the freeing of huge pages
220across all nodes in the memory policy of the task modifying ``nr_hugepages``.
221Any free huge pages on the selected nodes will be freed back to the kernel's
222normal page pool.
223
224Caveat: Shrinking the persistent huge page pool via ``nr_hugepages`` such that
225it becomes less than the number of huge pages in use will convert the balance
226of the in-use huge pages to surplus huge pages.  This will occur even if
227the number of surplus pages would exceed the overcommit value.  As long as
228this condition holds--that is, until ``nr_hugepages+nr_overcommit_hugepages`` is
229increased sufficiently, or the surplus huge pages go out of use and are freed--
230no more surplus huge pages will be allowed to be allocated.
231
232With support for multiple huge page pools at run-time available, much of
233the huge page userspace interface in ``/proc/sys/vm`` has been duplicated in
234sysfs.
235The ``/proc`` interfaces discussed above have been retained for backwards
236compatibility. The root huge page control directory in sysfs is::
237
238	/sys/kernel/mm/hugepages
239
240For each huge page size supported by the running kernel, a subdirectory
241will exist, of the form::
242
243	hugepages-${size}kB
244
245Inside each of these directories, the set of files contained in ``/proc``
246will exist.  In addition, two additional interfaces for demoting huge
247pages may exist::
248
249        demote
250        demote_size
251	nr_hugepages
252	nr_hugepages_mempolicy
253	nr_overcommit_hugepages
254	free_hugepages
255	resv_hugepages
256	surplus_hugepages
257
258The demote interfaces provide the ability to split a huge page into
259smaller huge pages.  For example, the x86 architecture supports both
2601GB and 2MB huge pages sizes.  A 1GB huge page can be split into 512
2612MB huge pages.  Demote interfaces are not available for the smallest
262huge page size.  The demote interfaces are:
263
264demote_size
265        is the size of demoted pages.  When a page is demoted a corresponding
266        number of huge pages of demote_size will be created.  By default,
267        demote_size is set to the next smaller huge page size.  If there are
268        multiple smaller huge page sizes, demote_size can be set to any of
269        these smaller sizes.  Only huge page sizes less than the current huge
270        pages size are allowed.
271
272demote
273        is used to demote a number of huge pages.  A user with root privileges
274        can write to this file.  It may not be possible to demote the
275        requested number of huge pages.  To determine how many pages were
276        actually demoted, compare the value of nr_hugepages before and after
277        writing to the demote interface.  demote is a write only interface.
278
279The interfaces which are the same as in ``/proc`` (all except demote and
280demote_size) function as described above for the default huge page-sized case.
281
282.. _mem_policy_and_hp_alloc:
283
284Interaction of Task Memory Policy with Huge Page Allocation/Freeing
285===================================================================
286
287Whether huge pages are allocated and freed via the ``/proc`` interface or
288the ``/sysfs`` interface using the ``nr_hugepages_mempolicy`` attribute, the
289NUMA nodes from which huge pages are allocated or freed are controlled by the
290NUMA memory policy of the task that modifies the ``nr_hugepages_mempolicy``
291sysctl or attribute.  When the ``nr_hugepages`` attribute is used, mempolicy
292is ignored.
293
294The recommended method to allocate or free huge pages to/from the kernel
295huge page pool, using the ``nr_hugepages`` example above, is::
296
297    numactl --interleave <node-list> echo 20 \
298				>/proc/sys/vm/nr_hugepages_mempolicy
299
300or, more succinctly::
301
302    numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy
303
304This will allocate or free ``abs(20 - nr_hugepages)`` to or from the nodes
305specified in <node-list>, depending on whether number of persistent huge pages
306is initially less than or greater than 20, respectively.  No huge pages will be
307allocated nor freed on any node not included in the specified <node-list>.
308
309When adjusting the persistent hugepage count via ``nr_hugepages_mempolicy``, any
310memory policy mode--bind, preferred, local or interleave--may be used.  The
311resulting effect on persistent huge page allocation is as follows:
312
313#. Regardless of mempolicy mode [see
314   Documentation/admin-guide/mm/numa_memory_policy.rst],
315   persistent huge pages will be distributed across the node or nodes
316   specified in the mempolicy as if "interleave" had been specified.
317   However, if a node in the policy does not contain sufficient contiguous
318   memory for a huge page, the allocation will not "fallback" to the nearest
319   neighbor node with sufficient contiguous memory.  To do this would cause
320   undesirable imbalance in the distribution of the huge page pool, or
321   possibly, allocation of persistent huge pages on nodes not allowed by
322   the task's memory policy.
323
324#. One or more nodes may be specified with the bind or interleave policy.
325   If more than one node is specified with the preferred policy, only the
326   lowest numeric id will be used.  Local policy will select the node where
327   the task is running at the time the nodes_allowed mask is constructed.
328   For local policy to be deterministic, the task must be bound to a cpu or
329   cpus in a single node.  Otherwise, the task could be migrated to some
330   other node at any time after launch and the resulting node will be
331   indeterminate.  Thus, local policy is not very useful for this purpose.
332   Any of the other mempolicy modes may be used to specify a single node.
333
334#. The nodes allowed mask will be derived from any non-default task mempolicy,
335   whether this policy was set explicitly by the task itself or one of its
336   ancestors, such as numactl.  This means that if the task is invoked from a
337   shell with non-default policy, that policy will be used.  One can specify a
338   node list of "all" with numactl --interleave or --membind [-m] to achieve
339   interleaving over all nodes in the system or cpuset.
340
341#. Any task mempolicy specified--e.g., using numactl--will be constrained by
342   the resource limits of any cpuset in which the task runs.  Thus, there will
343   be no way for a task with non-default policy running in a cpuset with a
344   subset of the system nodes to allocate huge pages outside the cpuset
345   without first moving to a cpuset that contains all of the desired nodes.
346
347#. Boot-time huge page allocation attempts to distribute the requested number
348   of huge pages over all on-lines nodes with memory.
349
350Per Node Hugepages Attributes
351=============================
352
353A subset of the contents of the root huge page control directory in sysfs,
354described above, will be replicated under each the system device of each
355NUMA node with memory in::
356
357	/sys/devices/system/node/node[0-9]*/hugepages/
358
359Under this directory, the subdirectory for each supported huge page size
360contains the following attribute files::
361
362	nr_hugepages
363	free_hugepages
364	surplus_hugepages
365
366The free\_' and surplus\_' attribute files are read-only.  They return the number
367of free and surplus [overcommitted] huge pages, respectively, on the parent
368node.
369
370The ``nr_hugepages`` attribute returns the total number of huge pages on the
371specified node.  When this attribute is written, the number of persistent huge
372pages on the parent node will be adjusted to the specified value, if sufficient
373resources exist, regardless of the task's mempolicy or cpuset constraints.
374
375Note that the number of overcommit and reserve pages remain global quantities,
376as we don't know until fault time, when the faulting task's mempolicy is
377applied, from which node the huge page allocation will be attempted.
378
379.. _using_huge_pages:
380
381Using Huge Pages
382================
383
384If the user applications are going to request huge pages using mmap system
385call, then it is required that system administrator mount a file system of
386type hugetlbfs::
387
388  mount -t hugetlbfs \
389	-o uid=<value>,gid=<value>,mode=<value>,pagesize=<value>,size=<value>,\
390	min_size=<value>,nr_inodes=<value> none /mnt/huge
391
392This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
393``/mnt/huge``.  Any file created on ``/mnt/huge`` uses huge pages.
394
395The ``uid`` and ``gid`` options sets the owner and group of the root of the
396file system.  By default the ``uid`` and ``gid`` of the current process
397are taken.
398
399The ``mode`` option sets the mode of root of file system to value & 01777.
400This value is given in octal. By default the value 0755 is picked.
401
402If the platform supports multiple huge page sizes, the ``pagesize`` option can
403be used to specify the huge page size and associated pool. ``pagesize``
404is specified in bytes. If ``pagesize`` is not specified the platform's
405default huge page size and associated pool will be used.
406
407The ``size`` option sets the maximum value of memory (huge pages) allowed
408for that filesystem (``/mnt/huge``). The ``size`` option can be specified
409in bytes, or as a percentage of the specified huge page pool (``nr_hugepages``).
410The size is rounded down to HPAGE_SIZE boundary.
411
412The ``min_size`` option sets the minimum value of memory (huge pages) allowed
413for the filesystem. ``min_size`` can be specified in the same way as ``size``,
414either bytes or a percentage of the huge page pool.
415At mount time, the number of huge pages specified by ``min_size`` are reserved
416for use by the filesystem.
417If there are not enough free huge pages available, the mount will fail.
418As huge pages are allocated to the filesystem and freed, the reserve count
419is adjusted so that the sum of allocated and reserved huge pages is always
420at least ``min_size``.
421
422The option ``nr_inodes`` sets the maximum number of inodes that ``/mnt/huge``
423can use.
424
425If the ``size``, ``min_size`` or ``nr_inodes`` option is not provided on
426command line then no limits are set.
427
428For ``pagesize``, ``size``, ``min_size`` and ``nr_inodes`` options, you can
429use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo.
430For example, size=2K has the same meaning as size=2048.
431
432While read system calls are supported on files that reside on hugetlb
433file systems, write system calls are not.
434
435Regular chown, chgrp, and chmod commands (with right permissions) could be
436used to change the file attributes on hugetlbfs.
437
438Also, it is important to note that no such mount command is required if
439applications are going to use only shmat/shmget system calls or mmap with
440MAP_HUGETLB.  For an example of how to use mmap with MAP_HUGETLB see
441:ref:`map_hugetlb <map_hugetlb>` below.
442
443Users who wish to use hugetlb memory via shared memory segment should be
444members of a supplementary group and system admin needs to configure that gid
445into ``/proc/sys/vm/hugetlb_shm_group``.  It is possible for same or different
446applications to use any combination of mmaps and shm* calls, though the mount of
447filesystem will be required for using mmap calls without MAP_HUGETLB.
448
449Syscalls that operate on memory backed by hugetlb pages only have their lengths
450aligned to the native page size of the processor; they will normally fail with
451errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
452not hugepage aligned.  For example, munmap(2) will fail if memory is backed by
453a hugetlb page and the length is smaller than the hugepage size.
454
455
456Examples
457========
458
459.. _map_hugetlb:
460
461``map_hugetlb``
462	see tools/testing/selftests/mm/map_hugetlb.c
463
464``hugepage-shm``
465	see tools/testing/selftests/mm/hugepage-shm.c
466
467``hugepage-mmap``
468	see tools/testing/selftests/mm/hugepage-mmap.c
469
470The `libhugetlbfs`_  library provides a wide range of userspace tools
471to help with huge page usability, environment setup, and control.
472
473.. _libhugetlbfs: https://github.com/libhugetlbfs/libhugetlbfs
474