path: root/Documentation/vm/hugetlbpage.txt

The intent of this file is to give a brief summary of hugetlbpage support in
the Linux kernel.  This support is built on top of multiple page size support
that is provided by most modern architectures.  For example, x86 CPUs normally
support 4K and 2M (1G if architecturally supported) page sizes, ia64
architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
256M and ppc64 supports 4K and 16M.  A TLB is a cache of virtual-to-physical
translations.  Typically this is a very scarce resource on processor.
Operating systems try to make best use of limited number of TLB resources.
This optimization is more critical now as bigger and bigger physical memories
(several GBs) are more readily available.

Users can use the huge page support in Linux kernel by either using the mmap
system call or standard SYSV shared memory system calls (shmget, shmat).

First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
(present under "File systems") and CONFIG_HUGETLB_PAGE (selected
automatically when CONFIG_HUGETLBFS is selected) configuration
options.

The /proc/meminfo file provides information about the total number of
persistent hugetlb pages in the kernel's huge page pool.  It also displays
information about the number of free, reserved and surplus huge pages and the
default huge page size.  The huge page size is needed for generating the
proper alignment and size of the arguments to system calls that map huge page
regions.

The output of "cat /proc/meminfo" will include lines like:

.....
HugePages_Total: vvv
HugePages_Free:  www
HugePages_Rsvd:  xxx
HugePages_Surp:  yyy
Hugepagesize:    zzz kB

where:
HugePages_Total is the size of the pool of huge pages.
HugePages_Free  is the number of huge pages in the pool that are not yet
                allocated.
HugePages_Rsvd  is short for "reserved," and is the number of huge pages for
                which a commitment to allocate from the pool has been made,
                but no allocation has yet been made.  Reserved huge pages
                guarantee that an application will be able to allocate a
                huge page from the pool of huge pages at fault time.
HugePages_Surp  is short for "surplus," and is the number of huge pages in
                the pool above the value in /proc/sys/vm/nr_hugepages. The
                maximum number of surplus huge pages is controlled by
                /proc/sys/vm/nr_overcommit_hugepages.

/proc/filesystems should also show a filesystem of type "hugetlbfs" configured
in the kernel.

/proc/sys/vm/nr_hugepages indicates the current number of "persistent" huge
pages in the kernel's huge page pool.  "Persistent" huge pages will be
returned to the huge page pool when freed by a task.  A user with root
privileges can dynamically allocate more or free some persistent huge pages
by increasing or decreasing the value of 'nr_hugepages'.

Pages that are used as huge pages are reserved inside the kernel and cannot
be used for other purposes.  Huge pages cannot be swapped out under
memory pressure.

Once a number of huge pages have been pre-allocated to the kernel huge page
pool, a user with appropriate privilege can use either the mmap system call
or shared memory system calls to use the huge pages.  See the discussion of
Using Huge Pages, below.

The administrator can allocate persistent huge pages on the kernel boot
command line by specifying the "hugepages=N" parameter, where 'N' = the
number of huge pages requested.  This is the most reliable method of
allocating huge pages as memory has not yet become fragmented.

Some platforms support multiple huge page sizes.  To allocate huge pages
of a specific size, one must precede the huge pages boot command parameters
with a huge page size selection parameter "hugepagesz=<size>".  <size> must
be specified in bytes with optional scale suffix [kKmMgG].  The default huge
page size may be selected with the "default_hugepagesz=<size>" boot parameter.

When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages
indicates the current number of pre-allocated huge pages of the default size.
Thus, one can use the following command to dynamically allocate/deallocate
default sized persistent huge pages:

	echo 20 > /proc/sys/vm/nr_hugepages

This command will try to adjust the number of default sized huge pages in the
huge page pool to 20, allocating or freeing huge pages, as required.

On a NUMA platform, the kernel will attempt to distribute the huge page pool
over all the set of allowed nodes specified by the NUMA memory policy of the
task that modifies nr_hugepages.  The default for the allowed nodes--when the
task has default memory policy--is all on-line nodes with memory.  Allowed
nodes with insufficient available, contiguous memory for a huge page will be
silently skipped when allocating persistent huge pages.  See the discussion
below of the interaction of task memory policy, cpusets and per node attributes
with the allocation and freeing of persistent huge pages.

The success or failure of huge page allocation depends on the amount of
physically contiguous memory that is present in system at the time of the
allocation attempt.  If the kernel is unable to allocate huge pages from
some nodes in a NUMA system, it will attempt to make up the difference by
allocating extra pages on other nodes with sufficient available contiguous
memory, if any.

System administrators may want to put this command in one of the local rc
init files.  This will enable the kernel to allocate huge pages early in
the boot process when the possibility of getting physical contiguous pages
is still very high.  Administrators can verify the number of huge pages
actually allocated by checking the sysctl or meminfo.  To check the per node
distribution of huge pages in a NUMA system, use:

	cat /sys/devices/system/node/node*/meminfo | fgrep Huge

/proc/sys/vm/nr_overcommit_hugepages specifies how large the pool of
huge pages can grow, if more huge pages than /proc/sys/vm/nr_hugepages are
requested by applications.  Writing any non-zero value into this file
indicates that the hugetlb subsystem is allowed to try to obtain that
number of "surplus" huge pages from the kernel's normal page pool, when the
persistent huge page pool is exhausted. As these surplus huge pages become
unused, they are freed back to the kernel's normal page pool.

When increasing the huge page pool size via nr_hugepages, any existing surplus
pages will first be promoted to persistent huge pages.  Then, additional
huge pages will be allocated, if necessary and if possible, to fulfill
the new persistent huge page pool size.

The administrator may shrink the pool of persistent huge pages for
the default huge page size by setting the nr_hugepages sysctl to a
smaller value.  The kernel will attempt to balance the freeing of huge pages
across all nodes in the memory policy of the task modifying nr_hugepages.
Any free huge pages on the selected nodes will be freed back to the kernel's
normal page pool.

Caveat: Shrinking the persistent huge page pool via nr_hugepages such that
it becomes less than the number of huge pages in use will convert the balance
of the in-use huge pages to surplus huge pages.  This will occur even if
the number of surplus pages it would exceed the overcommit value.  As long as
this condition holds--that is, until nr_hugepages+nr_overcommit_hugepages is
increased sufficiently, or the surplus huge pages go out of use and are freed--
no more surplus huge pages will be allowed to be allocated.

With support for multiple huge page pools at run-time available, much of
the huge page userspace interface in /proc/sys/vm has been duplicated in sysfs.
The /proc interfaces discussed above have been retained for backwards
compatibility. The root huge page control directory in sysfs is:

	/sys/kernel/mm/hugepages

For each huge page size supported by the running kernel, a subdirectory
will exist, of the form:

	hugepages-${size}kB

Inside each of these directories, the same set of files will exist:

	nr_hugepages
	nr_hugepages_mempolicy
	nr_overcommit_hugepages
	free_hugepages
	resv_hugepages
	surplus_hugepages

which function as described above for the default huge page-sized case.


Interaction of Task Memory Policy with Huge Page Allocation/Freeing
===================================================================

Whether huge pages are allocated and freed via the /proc interface or
the /sysfs interface using the nr_hugepages_mempolicy attribute, the NUMA
nodes from which huge pages are allocated or freed are controlled by the
NUMA memory policy of the task that modifies the nr_hugepages_mempolicy
sysctl or attribute.  When the nr_hugepages attribute is used, mempolicy
is ignored.

The recommended method to allocate or free huge pages to/from the kernel
huge page pool, using the nr_hugepages example above, is:

    numactl --interleave <node-list> echo 20 \
				>/proc/sys/vm/nr_hugepages_mempolicy

or, more succinctly:

    numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy

This will allocate or free abs(20 - nr_hugepages) to or from the nodes
specified in <node-list>, depending on whether number of persistent huge pages
is initially less than or greater than 20, respectively.  No huge pages will be
allocated nor freed on any node not included in the specified <node-list>.

When adjusting the persistent hugepage count via nr_hugepages_mempolicy, any
memory policy mode--bind, preferred, local or interleave--may be used.  The
resulting effect on persistent huge page allocation is as follows:

1) Regardless of mempolicy mode [see Documentation/vm/numa_memory_policy.txt],
   persistent huge pages will be distributed across the node or nodes
   specified in the mempolicy as if "interleave" had been specified.
   However, if a node in the policy does not contain sufficient contiguous
   memory for a huge page, the allocation will not "fallback" to the nearest
   neighbor node with sufficient contiguous memory.  To do this would cause
   undesirable imbalance in the distribution of the huge page pool, or
   possibly, allocation of persistent huge pages on nodes not allowed by
   the task's memory policy.

2) One or more nodes may be specified with the bind or interleave policy.
   If more than one node is specified with the preferred policy, only the
   lowest numeric id will be used.  Local policy will select the node where
   the task is running at the time the nodes_allowed mask is constructed.
   For local policy to be deterministic, the task must be bound to a cpu or
   cpus in a single node.  Otherwise, the task could be migrated to some
   other node at any time after launch and the resulting node will be
   indeterminate.  Thus, local policy is not very useful for this purpose.
   Any of the other mempolicy modes may be used to specify a single node.

3) The nodes allowed mask will be derived from any non-default task mempolicy,
   whether this policy was set explicitly by the task itself or one of its
   ancestors, such as numactl.  This means that if the task is invoked from a
   shell with non-default policy, that policy will be used.  One can specify a
   node list of "all" with numactl --interleave or --membind [-m] to achieve
   interleaving over all nodes in the system or cpuset.

4) Any task mempolicy specified--e.g., using numactl--will be constrained by
   the resource limits of any cpuset in which the task runs.  Thus, there will
   be no way for a task with non-default policy running in a cpuset with a
   subset of the system nodes to allocate huge pages outside the cpuset
   without first moving to a cpuset that contains all of the desired nodes.

5) Boot-time huge page allocation attempts to distribute the requested number
   of huge pages over all on-lines nodes with memory.

Per Node Hugepages Attributes
=============================

A subset of the contents of the root huge page control directory in sysfs,
described above, will be replicated under each the system device of each
NUMA node with memory in:

	/sys/devices/system/node/node[0-9]*/hugepages/

Under this directory, the subdirectory for each supported huge page size
contains the following attribute files:

	nr_hugepages
	free_hugepages
	surplus_hugepages

The free_' and surplus_' attribute files are read-only.  They return the number
of free and surplus [overcommitted] huge pages, respectively, on the parent
node.

The nr_hugepages attribute returns the total number of huge pages on the
specified node.  When this attribute is written, the number of persistent huge
pages on the parent node will be adjusted to the specified value, if sufficient
resources exist, regardless of the task's mempolicy or cpuset constraints.

Note that the number of overcommit and reserve pages remain global quantities,
as we don't know until fault time, when the faulting task's mempolicy is
applied, from which node the huge page allocation will be attempted.


Using Huge Pages
================

If the user applications are going to request huge pages using mmap system
call, then it is required that system administrator mount a file system of
type hugetlbfs:

  mount -t hugetlbfs \
	-o uid=<value>,gid=<value>,mode=<value>,pagesize=<value>,size=<value>,\
	min_size=<value>,nr_inodes=<value> none /mnt/huge

This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
/mnt/huge.  Any files created on /mnt/huge uses huge pages.  The uid and gid
options sets the owner and group of the root of the file system.  By default
the uid and gid of the current process are taken.  The mode option sets the
mode of root of file system to value & 01777.  This value is given in octal.
By default the value 0755 is picked. If the platform supports multiple huge
page sizes, the pagesize option can be used to specify the huge page size and
associated pool.  pagesize is specified in bytes.  If pagesize is not specified
the platform's default huge page size and associated pool will be used. The
size option sets the maximum value of memory (huge pages) allowed for that
filesystem (/mnt/huge).  The size option can be specified in bytes, or as a
percentage of the specified huge page pool (nr_hugepages).  The size is
rounded down to HPAGE_SIZE boundary.  The min_size option sets the minimum
value of memory (huge pages) allowed for the filesystem.  min_size can be
specified in the same way as size, either bytes or a percentage of the
huge page pool.  At mount time, the number of huge pages specified by
min_size are reserved for use by the filesystem.  If there are not enough
free huge pages available, the mount will fail.  As huge pages are allocated
to the filesystem and freed, the reserve count is adjusted so that the sum
of allocated and reserved huge pages is always at least min_size.  The option
nr_inodes sets the maximum number of inodes that /mnt/huge can use.  If the
size, min_size or nr_inodes option is not provided on command line then
no limits are set.  For pagesize, size, min_size and nr_inodes options, you
can use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo. For example, size=2K
has the same meaning as size=2048.

While read system calls are supported on files that reside on hugetlb
file systems, write system calls are not.

Regular chown, chgrp, and chmod commands (with right permissions) could be
used to change the file attributes on hugetlbfs.

Also, it is important to note that no such mount command is required if
applications are going to use only shmat/shmget system calls or mmap with
MAP_HUGETLB.  For an example of how to use mmap with MAP_HUGETLB see map_hugetlb
below.

Users who wish to use hugetlb memory via shared memory segment should be a
member of a supplementary group and system admin needs to configure that gid
into /proc/sys/vm/hugetlb_shm_group.  It is possible for same or different
applications to use any combination of mmaps and shm* calls, though the mount of
filesystem will be required for using mmap calls without MAP_HUGETLB.

Syscalls that operate on memory backed by hugetlb pages only have their lengths
aligned to the native page size of the processor; they will normally fail with
errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
not hugepage aligned.  For example, munmap(2) will fail if memory is backed by
a hugetlb page and the length is smaller than the hugepage size.


Examples
========

1) map_hugetlb: see tools/testing/selftests/vm/map_hugetlb.c

2) hugepage-shm:  see tools/testing/selftests/vm/hugepage-shm.c

3) hugepage-mmap:  see tools/testing/selftests/vm/hugepage-mmap.c

4) The libhugetlbfs (https://github.com/libhugetlbfs/libhugetlbfs) library
   provides a wide range of userspace tools to help with huge page usability,
   environment setup, and control.

Kernel development regression testing
=====================================

The most complete set of hugetlb tests are in the libhugetlbfs repository.
If you modify any hugetlb related code, use the libhugetlbfs test suite
to check for regressions.  In addition, if you add any new hugetlb
functionality, please add appropriate tests to libhugetlbfs.