Documentation / vm / hugetlbpage.txt

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Based on kernel version 3.9. Page generated on 2013-05-02 23:17 EST.

	
	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, i386
	architecture supports 4K and 4M (2M in PAE mode) 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 specifed--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>,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 & 0777.  This value is given in octal.
	By default the value 0755 is picked. The size option sets the maximum value of
	memory (huge pages) allowed for that filesystem (/mnt/huge). The size is
	rounded down to HPAGE_SIZE.  The option nr_inodes sets the maximum number of
	inodes that /mnt/huge can use.  If the size or nr_inodes option is not
	provided on command line then no limits are set.  For 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 the
	applications are going to use only shmat/shmget system calls or mmap with
	MAP_HUGETLB.  Users who wish to use hugetlb page 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.  For an example of how to use mmap with MAP_HUGETLB see
	map_hugetlb.c.
	
	*******************************************************************
	
	/*
	 * 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
	 */

• [ vm ]
• 00-INDEX
• active_mm.txt
• balance
• cleancache.txt
• frontswap.txt
• highmem.txt
• hugetlbpage.txt
• hwpoison.txt
• ksm.txt
• locking
• numa
• numa_memory_policy.txt
• overcommit-accounting
• page_migration
• pagemap.txt
• slub.txt
• transhuge.txt
• unevictable-lru.txt
•