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diff --git a/Documentation/vm/hugetlbpage.rst b/Documentation/vm/hugetlbpage.rst new file mode 100644 index 000000000000..a5da14b05b4b --- /dev/null +++ b/Documentation/vm/hugetlbpage.rst @@ -0,0 +1,386 @@ +.. _hugetlbpage: + +============= +HugeTLB Pages +============= + +Overview +======== + +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 +default huge page size and information about the number of free, reserved +and surplus huge pages in the pool of huge pages of default 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: uuu + HugePages_Free: vvv + HugePages_Rsvd: www + HugePages_Surp: xxx + Hugepagesize: yyy kB + Hugetlb: 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``. +Hugepagesize + is the default hugepage size (in Kb). +Hugetlb + is the total amount of memory (in kB), consumed by huge + pages of all sizes. + If huge pages of different sizes are in use, this number + will exceed HugePages_Total \* Hugepagesize. To get more + detailed information, please, refer to + ``/sys/kernel/mm/hugepages`` (described below). + + +``/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: + +#. Regardless of mempolicy mode [see Documentation/vm/numa_memory_policy.rst], + 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. + +#. 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. + +#. 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. + +#. 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. + +#. 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 +:ref:`map_hugetlb <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 +======== + +.. _map_hugetlb: + +``map_hugetlb`` + see tools/testing/selftests/vm/map_hugetlb.c + +``hugepage-shm`` + see tools/testing/selftests/vm/hugepage-shm.c + +``hugepage-mmap`` + see tools/testing/selftests/vm/hugepage-mmap.c + +The `libhugetlbfs`_ library provides a wide range of userspace tools +to help with huge page usability, environment setup, and control. + +.. _libhugetlbfs: https://github.com/libhugetlbfs/libhugetlbfs + +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. |