1 =============
2 HugeTLB Pages
3 =============
4
5 Overview
6 ========
7
8 The intent of this file is to give a brief summary of hugetlbpage support in
9 the Linux kernel. This support is built on top of multiple page size support
10 that is provided by most modern architectures. For example, x86 CPUs normally
11 support 4K and 2M (1G if architecturally supported) page sizes, ia64
12 architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
13 256M and ppc64 supports 4K and 16M. A TLB is a cache of virtual-to-physical
14 translations. Typically this is a very scarce resource on processor.
15 Operating systems try to make best use of limited number of TLB resources.
16 This optimization is more critical now as bigger and bigger physical memories
17 (several GBs) are more readily available.
18
19 Users can use the huge page support in Linux kernel by either using the mmap
20 system call or standard SYSV shared memory system calls (shmget, shmat).
21
22 First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
23 (present under "File systems") and CONFIG_HUGETLB_PAGE (selected
24 automatically when CONFIG_HUGETLBFS is selected) configuration
25 options.
26
27 The ``/proc/meminfo`` file provides information about the total number of
28 persistent hugetlb pages in the kernel's huge page pool. It also displays
29 default huge page size and information about the number of free, reserved
30 and surplus huge pages in the pool of huge pages of default size.
31 The huge page size is needed for generating the proper alignment and
32 size of the arguments to system calls that map huge page regions.
33
34 The 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
43 where:
44
45 HugePages_Total
46 is the size of the pool of huge pages.
47 HugePages_Free
48 is the number of huge pages in the pool that are not yet
49 allocated.
50 HugePages_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.
56 HugePages_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.
65 Hugepagesize
66 is the default hugepage size (in kB).
67 Hugetlb
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"
77 configured in the kernel.
78
79 ``/proc/sys/vm/nr_hugepages`` indicates the current number of "persistent" huge
80 pages in the kernel's huge page pool. "Persistent" huge pages will be
81 returned to the huge page pool when freed by a task. A user with root
82 privileges can dynamically allocate more or free some persistent huge pages
83 by increasing or decreasing the value of ``nr_hugepages``.
84
85 Note: When the feature of freeing unused vmemmap pages associated with each
86 hugetlb page is enabled, we can fail to free the huge pages triggered by
87 the user when the system is under memory pressure. Please try again later.
88
89 Pages that are used as huge pages are reserved inside the kernel and cannot
90 be used for other purposes. Huge pages cannot be swapped out under
91 memory pressure.
92
93 Once a number of huge pages have been pre-allocated to the kernel huge page
94 pool, a user with appropriate privilege can use either the mmap system call
95 or shared memory system calls to use the huge pages. See the discussion of
96 :ref:`Using Huge Pages <using_huge_pages>`, below.
97
98 The administrator can allocate persistent huge pages on the kernel boot
99 command line by specifying the "hugepages=N" parameter, where 'N' = the
100 number of huge pages requested. This is the most reliable method of
101 allocating huge pages as memory has not yet become fragmented.
102
103 Some platforms support multiple huge page sizes. To allocate huge pages
104 of a specific size, one must precede the huge pages boot command parameters
105 with a huge page size selection parameter "hugepagesz=<size>". <size> must
106 be specified in bytes with optional scale suffix [kKmMgG]. The default huge
107 page size may be selected with the "default_hugepagesz=<size>" boot parameter.
108
109 Hugetlb boot command line parameter semantics
110
111 hugepagesz
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.
122 hugepages
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
149 default_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.
164 hugetlb_free_vmemmap
165 When CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is set, this enables HugeTLB
166 Vmemmap Optimization (HVO).
167
168 When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
169 indicates the current number of pre-allocated huge pages of the default size.
170 Thus, one can use the following command to dynamically allocate/deallocate
171 default sized persistent huge pages::
172
173 echo 20 > /proc/sys/vm/nr_hugepages
174
175 This command will try to adjust the number of default sized huge pages in the
176 huge page pool to 20, allocating or freeing huge pages, as required.
177
178 On a NUMA platform, the kernel will attempt to distribute the huge page pool
179 over all the set of allowed nodes specified by the NUMA memory policy of the
180 task that modifies ``nr_hugepages``. The default for the allowed nodes--when the
181 task has default memory policy--is all on-line nodes with memory. Allowed
182 nodes with insufficient available, contiguous memory for a huge page will be
183 silently skipped when allocating persistent huge pages. See the
184 :ref:`discussion below <mem_policy_and_hp_alloc>`
185 of the interaction of task memory policy, cpusets and per node attributes
186 with the allocation and freeing of persistent huge pages.
187
188 The success or failure of huge page allocation depends on the amount of
189 physically contiguous memory that is present in system at the time of the
190 allocation attempt. If the kernel is unable to allocate huge pages from
191 some nodes in a NUMA system, it will attempt to make up the difference by
192 allocating extra pages on other nodes with sufficient available contiguous
193 memory, if any.
194
195 System administrators may want to put this command in one of the local rc
196 init files. This will enable the kernel to allocate huge pages early in
197 the boot process when the possibility of getting physical contiguous pages
198 is still very high. Administrators can verify the number of huge pages
199 actually allocated by checking the sysctl or meminfo. To check the per node
200 distribution 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
205 huge pages can grow, if more huge pages than ``/proc/sys/vm/nr_hugepages`` are
206 requested by applications. Writing any non-zero value into this file
207 indicates that the hugetlb subsystem is allowed to try to obtain that
208 number of "surplus" huge pages from the kernel's normal page pool, when the
209 persistent huge page pool is exhausted. As these surplus huge pages become
210 unused, they are freed back to the kernel's normal page pool.
211
212 When increasing the huge page pool size via ``nr_hugepages``, any existing
213 surplus pages will first be promoted to persistent huge pages. Then, additional
214 huge pages will be allocated, if necessary and if possible, to fulfill
215 the new persistent huge page pool size.
216
217 The administrator may shrink the pool of persistent huge pages for
218 the default huge page size by setting the ``nr_hugepages`` sysctl to a
219 smaller value. The kernel will attempt to balance the freeing of huge pages
220 across all nodes in the memory policy of the task modifying ``nr_hugepages``.
221 Any free huge pages on the selected nodes will be freed back to the kernel's
222 normal page pool.
223
224 Caveat: Shrinking the persistent huge page pool via ``nr_hugepages`` such that
225 it becomes less than the number of huge pages in use will convert the balance
226 of the in-use huge pages to surplus huge pages. This will occur even if
227 the number of surplus pages would exceed the overcommit value. As long as
228 this condition holds--that is, until ``nr_hugepages+nr_overcommit_hugepages`` is
229 increased sufficiently, or the surplus huge pages go out of use and are freed--
230 no more surplus huge pages will be allowed to be allocated.
231
232 With support for multiple huge page pools at run-time available, much of
233 the huge page userspace interface in ``/proc/sys/vm`` has been duplicated in
234 sysfs.
235 The ``/proc`` interfaces discussed above have been retained for backwards
236 compatibility. The root huge page control directory in sysfs is::
237
238 /sys/kernel/mm/hugepages
239
240 For each huge page size supported by the running kernel, a subdirectory
241 will exist, of the form::
242
243 hugepages-${size}kB
244
245 Inside each of these directories, the set of files contained in ``/proc``
246 will exist. In addition, two additional interfaces for demoting huge
247 pages 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
258 The demote interfaces provide the ability to split a huge page into
259 smaller huge pages. For example, the x86 architecture supports both
260 1GB and 2MB huge pages sizes. A 1GB huge page can be split into 512
261 2MB huge pages. Demote interfaces are not available for the smallest
262 huge page size. The demote interfaces are:
263
264 demote_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
272 demote
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
279 The interfaces which are the same as in ``/proc`` (all except demote and
280 demote_size) function as described above for the default huge page-sized case.
281
282 .. _mem_policy_and_hp_alloc:
283
284 Interaction of Task Memory Policy with Huge Page Allocation/Freeing
285 ===================================================================
286
287 Whether huge pages are allocated and freed via the ``/proc`` interface or
288 the ``/sysfs`` interface using the ``nr_hugepages_mempolicy`` attribute, the
289 NUMA nodes from which huge pages are allocated or freed are controlled by the
290 NUMA memory policy of the task that modifies the ``nr_hugepages_mempolicy``
291 sysctl or attribute. When the ``nr_hugepages`` attribute is used, mempolicy
292 is ignored.
293
294 The recommended method to allocate or free huge pages to/from the kernel
295 huge 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
300 or, more succinctly::
301
302 numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy
303
304 This will allocate or free ``abs(20 - nr_hugepages)`` to or from the nodes
305 specified in <node-list>, depending on whether number of persistent huge pages
306 is initially less than or greater than 20, respectively. No huge pages will be
307 allocated nor freed on any node not included in the specified <node-list>.
308
309 When adjusting the persistent hugepage count via ``nr_hugepages_mempolicy``, any
310 memory policy mode--bind, preferred, local or interleave--may be used. The
311 resulting 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
350 Per Node Hugepages Attributes
351 =============================
352
353 A subset of the contents of the root huge page control directory in sysfs,
354 described above, will be replicated under each the system device of each
355 NUMA node with memory in::
356
357 /sys/devices/system/node/node[0-9]*/hugepages/
358
359 Under this directory, the subdirectory for each supported huge page size
360 contains the following attribute files::
361
362 nr_hugepages
363 free_hugepages
364 surplus_hugepages
365
366 The free\_' and surplus\_' attribute files are read-only. They return the number
367 of free and surplus [overcommitted] huge pages, respectively, on the parent
368 node.
369
370 The ``nr_hugepages`` attribute returns the total number of huge pages on the
371 specified node. When this attribute is written, the number of persistent huge
372 pages on the parent node will be adjusted to the specified value, if sufficient
373 resources exist, regardless of the task's mempolicy or cpuset constraints.
374
375 Note that the number of overcommit and reserve pages remain global quantities,
376 as we don't know until fault time, when the faulting task's mempolicy is
377 applied, from which node the huge page allocation will be attempted.
378
379 The hugetlb may be migrated between the per-node hugepages pool in the following
380 scenarios: memory offline, memory failure, longterm pinning, syscalls(mbind,
381 migrate_pages and move_pages), alloc_contig_range() and alloc_contig_pages().
382 Now only memory offline, memory failure and syscalls allow fallbacking to allocate
383 a new hugetlb on a different node if the current node is unable to allocate during
384 hugetlb migration, that means these 3 cases can break the per-node hugepages pool.
385
386 .. _using_huge_pages:
387
388 Using Huge Pages
389 ================
390
391 If the user applications are going to request huge pages using mmap system
392 call, then it is required that system administrator mount a file system of
393 type hugetlbfs::
394
395 mount -t hugetlbfs \
396 -o uid=<value>,gid=<value>,mode=<value>,pagesize=<value>,size=<value>,\
397 min_size=<value>,nr_inodes=<value> none /mnt/huge
398
399 This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
400 ``/mnt/huge``. Any file created on ``/mnt/huge`` uses huge pages.
401
402 The ``uid`` and ``gid`` options sets the owner and group of the root of the
403 file system. By default the ``uid`` and ``gid`` of the current process
404 are taken.
405
406 The ``mode`` option sets the mode of root of file system to value & 01777.
407 This value is given in octal. By default the value 0755 is picked.
408
409 If the platform supports multiple huge page sizes, the ``pagesize`` option can
410 be used to specify the huge page size and associated pool. ``pagesize``
411 is specified in bytes. If ``pagesize`` is not specified the platform's
412 default huge page size and associated pool will be used.
413
414 The ``size`` option sets the maximum value of memory (huge pages) allowed
415 for that filesystem (``/mnt/huge``). The ``size`` option can be specified
416 in bytes, or as a percentage of the specified huge page pool (``nr_hugepages``).
417 The size is rounded down to HPAGE_SIZE boundary.
418
419 The ``min_size`` option sets the minimum value of memory (huge pages) allowed
420 for the filesystem. ``min_size`` can be specified in the same way as ``size``,
421 either bytes or a percentage of the huge page pool.
422 At mount time, the number of huge pages specified by ``min_size`` are reserved
423 for use by the filesystem.
424 If there are not enough free huge pages available, the mount will fail.
425 As huge pages are allocated to the filesystem and freed, the reserve count
426 is adjusted so that the sum of allocated and reserved huge pages is always
427 at least ``min_size``.
428
429 The option ``nr_inodes`` sets the maximum number of inodes that ``/mnt/huge``
430 can use.
431
432 If the ``size``, ``min_size`` or ``nr_inodes`` option is not provided on
433 command line then no limits are set.
434
435 For ``pagesize``, ``size``, ``min_size`` and ``nr_inodes`` options, you can
436 use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo.
437 For example, size=2K has the same meaning as size=2048.
438
439 While read system calls are supported on files that reside on hugetlb
440 file systems, write system calls are not.
441
442 Regular chown, chgrp, and chmod commands (with right permissions) could be
443 used to change the file attributes on hugetlbfs.
444
445 Also, it is important to note that no such mount command is required if
446 applications are going to use only shmat/shmget system calls or mmap with
447 MAP_HUGETLB. For an example of how to use mmap with MAP_HUGETLB see
448 :ref:`map_hugetlb <map_hugetlb>` below.
449
450 Users who wish to use hugetlb memory via shared memory segment should be
451 members of a supplementary group and system admin needs to configure that gid
452 into ``/proc/sys/vm/hugetlb_shm_group``. It is possible for same or different
453 applications to use any combination of mmaps and shm* calls, though the mount of
454 filesystem will be required for using mmap calls without MAP_HUGETLB.
455
456 Syscalls that operate on memory backed by hugetlb pages only have their lengths
457 aligned to the native page size of the processor; they will normally fail with
458 errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
459 not hugepage aligned. For example, munmap(2) will fail if memory is backed by
460 a hugetlb page and the length is smaller than the hugepage size.
461
462
463 Examples
464 ========
465
466 .. _map_hugetlb:
467
468 ``map_hugetlb``
469 see tools/testing/selftests/mm/map_hugetlb.c
470
471 ``hugepage-shm``
472 see tools/testing/selftests/mm/hugepage-shm.c
473
474 ``hugepage-mmap``
475 see tools/testing/selftests/mm/hugepage-mmap.c
476
477 The `libhugetlbfs`_ library provides a wide range of userspace tools
478 to help with huge page usability, environment setup, and control.
479
480 .. _libhugetlbfs: https://github.com/libhugetlbfs/libhugetlbfs
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