1.. _hugetlbpage:
2
3=============
4HugeTLB Pages
5=============
6
7Overview
8========
9
10The intent of this file is to give a brief summary of hugetlbpage support in
11the Linux kernel.  This support is built on top of multiple page size support
12that is provided by most modern architectures.  For example, x86 CPUs normally
13support 4K and 2M (1G if architecturally supported) page sizes, ia64
14architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
15256M and ppc64 supports 4K and 16M.  A TLB is a cache of virtual-to-physical
16translations.  Typically this is a very scarce resource on processor.
17Operating systems try to make best use of limited number of TLB resources.
18This optimization is more critical now as bigger and bigger physical memories
19(several GBs) are more readily available.
20
21Users can use the huge page support in Linux kernel by either using the mmap
22system call or standard SYSV shared memory system calls (shmget, shmat).
23
24First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
25(present under "File systems") and CONFIG_HUGETLB_PAGE (selected
26automatically when CONFIG_HUGETLBFS is selected) configuration
27options.
28
29The ``/proc/meminfo`` file provides information about the total number of
30persistent hugetlb pages in the kernel's huge page pool.  It also displays
31default huge page size and information about the number of free, reserved
32and surplus huge pages in the pool of huge pages of default size.
33The huge page size is needed for generating the proper alignment and
34size of the arguments to system calls that map huge page regions.
35
36The output of ``cat /proc/meminfo`` will include lines like::
37
38	HugePages_Total: uuu
39	HugePages_Free:  vvv
40	HugePages_Rsvd:  www
41	HugePages_Surp:  xxx
42	Hugepagesize:    yyy kB
43	Hugetlb:         zzz kB
44
45where:
46
47HugePages_Total
48	is the size of the pool of huge pages.
49HugePages_Free
50	is the number of huge pages in the pool that are not yet
51        allocated.
52HugePages_Rsvd
53	is short for "reserved," and is the number of huge pages for
54        which a commitment to allocate from the pool has been made,
55        but no allocation has yet been made.  Reserved huge pages
56        guarantee that an application will be able to allocate a
57        huge page from the pool of huge pages at fault time.
58HugePages_Surp
59	is short for "surplus," and is the number of huge pages in
60        the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
61        maximum number of surplus huge pages is controlled by
62        ``/proc/sys/vm/nr_overcommit_hugepages``.
63Hugepagesize
64	is the default hugepage size (in Kb).
65Hugetlb
66        is the total amount of memory (in kB), consumed by huge
67        pages of all sizes.
68        If huge pages of different sizes are in use, this number
69        will exceed HugePages_Total \* Hugepagesize. To get more
70        detailed information, please, refer to
71        ``/sys/kernel/mm/hugepages`` (described below).
72
73
74``/proc/filesystems`` should also show a filesystem of type "hugetlbfs"
75configured in the kernel.
76
77``/proc/sys/vm/nr_hugepages`` indicates the current number of "persistent" huge
78pages in the kernel's huge page pool.  "Persistent" huge pages will be
79returned to the huge page pool when freed by a task.  A user with root
80privileges can dynamically allocate more or free some persistent huge pages
81by increasing or decreasing the value of ``nr_hugepages``.
82
83Pages that are used as huge pages are reserved inside the kernel and cannot
84be used for other purposes.  Huge pages cannot be swapped out under
85memory pressure.
86
87Once a number of huge pages have been pre-allocated to the kernel huge page
88pool, a user with appropriate privilege can use either the mmap system call
89or shared memory system calls to use the huge pages.  See the discussion of
90:ref:`Using Huge Pages <using_huge_pages>`, below.
91
92The administrator can allocate persistent huge pages on the kernel boot
93command line by specifying the "hugepages=N" parameter, where 'N' = the
94number of huge pages requested.  This is the most reliable method of
95allocating huge pages as memory has not yet become fragmented.
96
97Some platforms support multiple huge page sizes.  To allocate huge pages
98of a specific size, one must precede the huge pages boot command parameters
99with a huge page size selection parameter "hugepagesz=<size>".  <size> must
100be specified in bytes with optional scale suffix [kKmMgG].  The default huge
101page size may be selected with the "default_hugepagesz=<size>" boot parameter.
102
103Hugetlb boot command line parameter semantics
104
105hugepagesz
106	Specify a huge page size.  Used in conjunction with hugepages
107	parameter to preallocate a number of huge pages of the specified
108	size.  Hence, hugepagesz and hugepages are typically specified in
109	pairs such as::
110
111		hugepagesz=2M hugepages=512
112
113	hugepagesz can only be specified once on the command line for a
114	specific huge page size.  Valid huge page sizes are architecture
115	dependent.
116hugepages
117	Specify the number of huge pages to preallocate.  This typically
118	follows a valid hugepagesz or default_hugepagesz parameter.  However,
119	if hugepages is the first or only hugetlb command line parameter it
120	implicitly specifies the number of huge pages of default size to
121	allocate.  If the number of huge pages of default size is implicitly
122	specified, it can not be overwritten by a hugepagesz,hugepages
123	parameter pair for the default size.
124
125	For example, on an architecture with 2M default huge page size::
126
127		hugepages=256 hugepagesz=2M hugepages=512
128
129	will result in 256 2M huge pages being allocated and a warning message
130	indicating that the hugepages=512 parameter is ignored.  If a hugepages
131	parameter is preceded by an invalid hugepagesz parameter, it will
132	be ignored.
133default_hugepagesz
134	pecify the default huge page size.  This parameter can
135	only be specified once on the command line.  default_hugepagesz can
136	optionally be followed by the hugepages parameter to preallocate a
137	specific number of huge pages of default size.  The number of default
138	sized huge pages to preallocate can also be implicitly specified as
139	mentioned in the hugepages section above.  Therefore, on an
140	architecture with 2M default huge page size::
141
142		hugepages=256
143		default_hugepagesz=2M hugepages=256
144		hugepages=256 default_hugepagesz=2M
145
146	will all result in 256 2M huge pages being allocated.  Valid default
147	huge page size is architecture dependent.
148
149When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
150indicates the current number of pre-allocated huge pages of the default size.
151Thus, one can use the following command to dynamically allocate/deallocate
152default sized persistent huge pages::
153
154	echo 20 > /proc/sys/vm/nr_hugepages
155
156This command will try to adjust the number of default sized huge pages in the
157huge page pool to 20, allocating or freeing huge pages, as required.
158
159On a NUMA platform, the kernel will attempt to distribute the huge page pool
160over all the set of allowed nodes specified by the NUMA memory policy of the
161task that modifies ``nr_hugepages``. The default for the allowed nodes--when the
162task has default memory policy--is all on-line nodes with memory.  Allowed
163nodes with insufficient available, contiguous memory for a huge page will be
164silently skipped when allocating persistent huge pages.  See the
165:ref:`discussion below <mem_policy_and_hp_alloc>`
166of the interaction of task memory policy, cpusets and per node attributes
167with the allocation and freeing of persistent huge pages.
168
169The success or failure of huge page allocation depends on the amount of
170physically contiguous memory that is present in system at the time of the
171allocation attempt.  If the kernel is unable to allocate huge pages from
172some nodes in a NUMA system, it will attempt to make up the difference by
173allocating extra pages on other nodes with sufficient available contiguous
174memory, if any.
175
176System administrators may want to put this command in one of the local rc
177init files.  This will enable the kernel to allocate huge pages early in
178the boot process when the possibility of getting physical contiguous pages
179is still very high.  Administrators can verify the number of huge pages
180actually allocated by checking the sysctl or meminfo.  To check the per node
181distribution of huge pages in a NUMA system, use::
182
183	cat /sys/devices/system/node/node*/meminfo | fgrep Huge
184
185``/proc/sys/vm/nr_overcommit_hugepages`` specifies how large the pool of
186huge pages can grow, if more huge pages than ``/proc/sys/vm/nr_hugepages`` are
187requested by applications.  Writing any non-zero value into this file
188indicates that the hugetlb subsystem is allowed to try to obtain that
189number of "surplus" huge pages from the kernel's normal page pool, when the
190persistent huge page pool is exhausted. As these surplus huge pages become
191unused, they are freed back to the kernel's normal page pool.
192
193When increasing the huge page pool size via ``nr_hugepages``, any existing
194surplus pages will first be promoted to persistent huge pages.  Then, additional
195huge pages will be allocated, if necessary and if possible, to fulfill
196the new persistent huge page pool size.
197
198The administrator may shrink the pool of persistent huge pages for
199the default huge page size by setting the ``nr_hugepages`` sysctl to a
200smaller value.  The kernel will attempt to balance the freeing of huge pages
201across all nodes in the memory policy of the task modifying ``nr_hugepages``.
202Any free huge pages on the selected nodes will be freed back to the kernel's
203normal page pool.
204
205Caveat: Shrinking the persistent huge page pool via ``nr_hugepages`` such that
206it becomes less than the number of huge pages in use will convert the balance
207of the in-use huge pages to surplus huge pages.  This will occur even if
208the number of surplus pages would exceed the overcommit value.  As long as
209this condition holds--that is, until ``nr_hugepages+nr_overcommit_hugepages`` is
210increased sufficiently, or the surplus huge pages go out of use and are freed--
211no more surplus huge pages will be allowed to be allocated.
212
213With support for multiple huge page pools at run-time available, much of
214the huge page userspace interface in ``/proc/sys/vm`` has been duplicated in
215sysfs.
216The ``/proc`` interfaces discussed above have been retained for backwards
217compatibility. The root huge page control directory in sysfs is::
218
219	/sys/kernel/mm/hugepages
220
221For each huge page size supported by the running kernel, a subdirectory
222will exist, of the form::
223
224	hugepages-${size}kB
225
226Inside each of these directories, the same set of files will exist::
227
228	nr_hugepages
229	nr_hugepages_mempolicy
230	nr_overcommit_hugepages
231	free_hugepages
232	resv_hugepages
233	surplus_hugepages
234
235which function as described above for the default huge page-sized case.
236
237.. _mem_policy_and_hp_alloc:
238
239Interaction of Task Memory Policy with Huge Page Allocation/Freeing
240===================================================================
241
242Whether huge pages are allocated and freed via the ``/proc`` interface or
243the ``/sysfs`` interface using the ``nr_hugepages_mempolicy`` attribute, the
244NUMA nodes from which huge pages are allocated or freed are controlled by the
245NUMA memory policy of the task that modifies the ``nr_hugepages_mempolicy``
246sysctl or attribute.  When the ``nr_hugepages`` attribute is used, mempolicy
247is ignored.
248
249The recommended method to allocate or free huge pages to/from the kernel
250huge page pool, using the ``nr_hugepages`` example above, is::
251
252    numactl --interleave <node-list> echo 20 \
253				>/proc/sys/vm/nr_hugepages_mempolicy
254
255or, more succinctly::
256
257    numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy
258
259This will allocate or free ``abs(20 - nr_hugepages)`` to or from the nodes
260specified in <node-list>, depending on whether number of persistent huge pages
261is initially less than or greater than 20, respectively.  No huge pages will be
262allocated nor freed on any node not included in the specified <node-list>.
263
264When adjusting the persistent hugepage count via ``nr_hugepages_mempolicy``, any
265memory policy mode--bind, preferred, local or interleave--may be used.  The
266resulting effect on persistent huge page allocation is as follows:
267
268#. Regardless of mempolicy mode [see
269   :ref:`Documentation/admin-guide/mm/numa_memory_policy.rst <numa_memory_policy>`],
270   persistent huge pages will be distributed across the node or nodes
271   specified in the mempolicy as if "interleave" had been specified.
272   However, if a node in the policy does not contain sufficient contiguous
273   memory for a huge page, the allocation will not "fallback" to the nearest
274   neighbor node with sufficient contiguous memory.  To do this would cause
275   undesirable imbalance in the distribution of the huge page pool, or
276   possibly, allocation of persistent huge pages on nodes not allowed by
277   the task's memory policy.
278
279#. One or more nodes may be specified with the bind or interleave policy.
280   If more than one node is specified with the preferred policy, only the
281   lowest numeric id will be used.  Local policy will select the node where
282   the task is running at the time the nodes_allowed mask is constructed.
283   For local policy to be deterministic, the task must be bound to a cpu or
284   cpus in a single node.  Otherwise, the task could be migrated to some
285   other node at any time after launch and the resulting node will be
286   indeterminate.  Thus, local policy is not very useful for this purpose.
287   Any of the other mempolicy modes may be used to specify a single node.
288
289#. The nodes allowed mask will be derived from any non-default task mempolicy,
290   whether this policy was set explicitly by the task itself or one of its
291   ancestors, such as numactl.  This means that if the task is invoked from a
292   shell with non-default policy, that policy will be used.  One can specify a
293   node list of "all" with numactl --interleave or --membind [-m] to achieve
294   interleaving over all nodes in the system or cpuset.
295
296#. Any task mempolicy specified--e.g., using numactl--will be constrained by
297   the resource limits of any cpuset in which the task runs.  Thus, there will
298   be no way for a task with non-default policy running in a cpuset with a
299   subset of the system nodes to allocate huge pages outside the cpuset
300   without first moving to a cpuset that contains all of the desired nodes.
301
302#. Boot-time huge page allocation attempts to distribute the requested number
303   of huge pages over all on-lines nodes with memory.
304
305Per Node Hugepages Attributes
306=============================
307
308A subset of the contents of the root huge page control directory in sysfs,
309described above, will be replicated under each the system device of each
310NUMA node with memory in::
311
312	/sys/devices/system/node/node[0-9]*/hugepages/
313
314Under this directory, the subdirectory for each supported huge page size
315contains the following attribute files::
316
317	nr_hugepages
318	free_hugepages
319	surplus_hugepages
320
321The free\_' and surplus\_' attribute files are read-only.  They return the number
322of free and surplus [overcommitted] huge pages, respectively, on the parent
323node.
324
325The ``nr_hugepages`` attribute returns the total number of huge pages on the
326specified node.  When this attribute is written, the number of persistent huge
327pages on the parent node will be adjusted to the specified value, if sufficient
328resources exist, regardless of the task's mempolicy or cpuset constraints.
329
330Note that the number of overcommit and reserve pages remain global quantities,
331as we don't know until fault time, when the faulting task's mempolicy is
332applied, from which node the huge page allocation will be attempted.
333
334.. _using_huge_pages:
335
336Using Huge Pages
337================
338
339If the user applications are going to request huge pages using mmap system
340call, then it is required that system administrator mount a file system of
341type hugetlbfs::
342
343  mount -t hugetlbfs \
344	-o uid=<value>,gid=<value>,mode=<value>,pagesize=<value>,size=<value>,\
345	min_size=<value>,nr_inodes=<value> none /mnt/huge
346
347This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
348``/mnt/huge``.  Any file created on ``/mnt/huge`` uses huge pages.
349
350The ``uid`` and ``gid`` options sets the owner and group of the root of the
351file system.  By default the ``uid`` and ``gid`` of the current process
352are taken.
353
354The ``mode`` option sets the mode of root of file system to value & 01777.
355This value is given in octal. By default the value 0755 is picked.
356
357If the platform supports multiple huge page sizes, the ``pagesize`` option can
358be used to specify the huge page size and associated pool. ``pagesize``
359is specified in bytes. If ``pagesize`` is not specified the platform's
360default huge page size and associated pool will be used.
361
362The ``size`` option sets the maximum value of memory (huge pages) allowed
363for that filesystem (``/mnt/huge``). The ``size`` option can be specified
364in bytes, or as a percentage of the specified huge page pool (``nr_hugepages``).
365The size is rounded down to HPAGE_SIZE boundary.
366
367The ``min_size`` option sets the minimum value of memory (huge pages) allowed
368for the filesystem. ``min_size`` can be specified in the same way as ``size``,
369either bytes or a percentage of the huge page pool.
370At mount time, the number of huge pages specified by ``min_size`` are reserved
371for use by the filesystem.
372If there are not enough free huge pages available, the mount will fail.
373As huge pages are allocated to the filesystem and freed, the reserve count
374is adjusted so that the sum of allocated and reserved huge pages is always
375at least ``min_size``.
376
377The option ``nr_inodes`` sets the maximum number of inodes that ``/mnt/huge``
378can use.
379
380If the ``size``, ``min_size`` or ``nr_inodes`` option is not provided on
381command line then no limits are set.
382
383For ``pagesize``, ``size``, ``min_size`` and ``nr_inodes`` options, you can
384use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo.
385For example, size=2K has the same meaning as size=2048.
386
387While read system calls are supported on files that reside on hugetlb
388file systems, write system calls are not.
389
390Regular chown, chgrp, and chmod commands (with right permissions) could be
391used to change the file attributes on hugetlbfs.
392
393Also, it is important to note that no such mount command is required if
394applications are going to use only shmat/shmget system calls or mmap with
395MAP_HUGETLB.  For an example of how to use mmap with MAP_HUGETLB see
396:ref:`map_hugetlb <map_hugetlb>` below.
397
398Users who wish to use hugetlb memory via shared memory segment should be
399members of a supplementary group and system admin needs to configure that gid
400into ``/proc/sys/vm/hugetlb_shm_group``.  It is possible for same or different
401applications to use any combination of mmaps and shm* calls, though the mount of
402filesystem will be required for using mmap calls without MAP_HUGETLB.
403
404Syscalls that operate on memory backed by hugetlb pages only have their lengths
405aligned to the native page size of the processor; they will normally fail with
406errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
407not hugepage aligned.  For example, munmap(2) will fail if memory is backed by
408a hugetlb page and the length is smaller than the hugepage size.
409
410
411Examples
412========
413
414.. _map_hugetlb:
415
416``map_hugetlb``
417	see tools/testing/selftests/vm/map_hugetlb.c
418
419``hugepage-shm``
420	see tools/testing/selftests/vm/hugepage-shm.c
421
422``hugepage-mmap``
423	see tools/testing/selftests/vm/hugepage-mmap.c
424
425The `libhugetlbfs`_  library provides a wide range of userspace tools
426to help with huge page usability, environment setup, and control.
427
428.. _libhugetlbfs: https://github.com/libhugetlbfs/libhugetlbfs
429