343 lines
17 KiB
Plaintext
343 lines
17 KiB
Plaintext
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The intent of this file is to give a brief summary of hugetlbpage support in
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the Linux kernel. This support is built on top of multiple page size support
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that is provided by most modern architectures. For example, x86 CPUs normally
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support 4K and 2M (1G if architecturally supported) page sizes, ia64
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architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
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256M and ppc64 supports 4K and 16M. A TLB is a cache of virtual-to-physical
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translations. Typically this is a very scarce resource on processor.
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Operating systems try to make best use of limited number of TLB resources.
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This optimization is more critical now as bigger and bigger physical memories
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(several GBs) are more readily available.
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Users can use the huge page support in Linux kernel by either using the mmap
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system call or standard SYSV shared memory system calls (shmget, shmat).
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First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
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(present under "File systems") and CONFIG_HUGETLB_PAGE (selected
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automatically when CONFIG_HUGETLBFS is selected) configuration
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options.
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The /proc/meminfo file provides information about the total number of
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persistent hugetlb pages in the kernel's huge page pool. It also displays
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information about the number of free, reserved and surplus huge pages and the
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default huge page size. The huge page size is needed for generating the
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proper alignment and size of the arguments to system calls that map huge page
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regions.
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The output of "cat /proc/meminfo" will include lines like:
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.....
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HugePages_Total: vvv
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HugePages_Free: www
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HugePages_Rsvd: xxx
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HugePages_Surp: yyy
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Hugepagesize: zzz kB
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where:
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HugePages_Total is the size of the pool of huge pages.
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HugePages_Free is the number of huge pages in the pool that are not yet
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allocated.
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HugePages_Rsvd is short for "reserved," and is the number of huge pages for
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which a commitment to allocate from the pool has been made,
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but no allocation has yet been made. Reserved huge pages
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guarantee that an application will be able to allocate a
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huge page from the pool of huge pages at fault time.
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HugePages_Surp is short for "surplus," and is the number of huge pages in
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the pool above the value in /proc/sys/vm/nr_hugepages. The
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maximum number of surplus huge pages is controlled by
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/proc/sys/vm/nr_overcommit_hugepages.
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/proc/filesystems should also show a filesystem of type "hugetlbfs" configured
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in the kernel.
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/proc/sys/vm/nr_hugepages indicates the current number of "persistent" huge
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pages in the kernel's huge page pool. "Persistent" huge pages will be
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returned to the huge page pool when freed by a task. A user with root
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privileges can dynamically allocate more or free some persistent huge pages
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by increasing or decreasing the value of 'nr_hugepages'.
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Pages that are used as huge pages are reserved inside the kernel and cannot
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be used for other purposes. Huge pages cannot be swapped out under
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memory pressure.
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Once a number of huge pages have been pre-allocated to the kernel huge page
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pool, a user with appropriate privilege can use either the mmap system call
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or shared memory system calls to use the huge pages. See the discussion of
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Using Huge Pages, below.
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The administrator can allocate persistent huge pages on the kernel boot
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command line by specifying the "hugepages=N" parameter, where 'N' = the
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number of huge pages requested. This is the most reliable method of
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allocating huge pages as memory has not yet become fragmented.
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Some platforms support multiple huge page sizes. To allocate huge pages
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of a specific size, one must precede the huge pages boot command parameters
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with a huge page size selection parameter "hugepagesz=<size>". <size> must
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be specified in bytes with optional scale suffix [kKmMgG]. The default huge
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page size may be selected with the "default_hugepagesz=<size>" boot parameter.
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When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages
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indicates the current number of pre-allocated huge pages of the default size.
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Thus, one can use the following command to dynamically allocate/deallocate
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default sized persistent huge pages:
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echo 20 > /proc/sys/vm/nr_hugepages
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This command will try to adjust the number of default sized huge pages in the
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huge page pool to 20, allocating or freeing huge pages, as required.
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On a NUMA platform, the kernel will attempt to distribute the huge page pool
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over all the set of allowed nodes specified by the NUMA memory policy of the
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task that modifies nr_hugepages. The default for the allowed nodes--when the
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task has default memory policy--is all on-line nodes with memory. Allowed
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nodes with insufficient available, contiguous memory for a huge page will be
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silently skipped when allocating persistent huge pages. See the discussion
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below of the interaction of task memory policy, cpusets and per node attributes
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with the allocation and freeing of persistent huge pages.
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The success or failure of huge page allocation depends on the amount of
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physically contiguous memory that is present in system at the time of the
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allocation attempt. If the kernel is unable to allocate huge pages from
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some nodes in a NUMA system, it will attempt to make up the difference by
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allocating extra pages on other nodes with sufficient available contiguous
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memory, if any.
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System administrators may want to put this command in one of the local rc
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init files. This will enable the kernel to allocate huge pages early in
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the boot process when the possibility of getting physical contiguous pages
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is still very high. Administrators can verify the number of huge pages
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actually allocated by checking the sysctl or meminfo. To check the per node
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distribution of huge pages in a NUMA system, use:
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cat /sys/devices/system/node/node*/meminfo | fgrep Huge
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/proc/sys/vm/nr_overcommit_hugepages specifies how large the pool of
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huge pages can grow, if more huge pages than /proc/sys/vm/nr_hugepages are
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requested by applications. Writing any non-zero value into this file
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indicates that the hugetlb subsystem is allowed to try to obtain that
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number of "surplus" huge pages from the kernel's normal page pool, when the
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persistent huge page pool is exhausted. As these surplus huge pages become
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unused, they are freed back to the kernel's normal page pool.
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When increasing the huge page pool size via nr_hugepages, any existing surplus
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pages will first be promoted to persistent huge pages. Then, additional
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huge pages will be allocated, if necessary and if possible, to fulfill
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the new persistent huge page pool size.
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The administrator may shrink the pool of persistent huge pages for
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the default huge page size by setting the nr_hugepages sysctl to a
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smaller value. The kernel will attempt to balance the freeing of huge pages
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across all nodes in the memory policy of the task modifying nr_hugepages.
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Any free huge pages on the selected nodes will be freed back to the kernel's
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normal page pool.
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Caveat: Shrinking the persistent huge page pool via nr_hugepages such that
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it becomes less than the number of huge pages in use will convert the balance
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of the in-use huge pages to surplus huge pages. This will occur even if
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the number of surplus pages it would exceed the overcommit value. As long as
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this condition holds--that is, until nr_hugepages+nr_overcommit_hugepages is
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increased sufficiently, or the surplus huge pages go out of use and are freed--
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no more surplus huge pages will be allowed to be allocated.
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With support for multiple huge page pools at run-time available, much of
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the huge page userspace interface in /proc/sys/vm has been duplicated in sysfs.
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The /proc interfaces discussed above have been retained for backwards
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compatibility. The root huge page control directory in sysfs is:
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/sys/kernel/mm/hugepages
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For each huge page size supported by the running kernel, a subdirectory
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will exist, of the form:
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hugepages-${size}kB
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Inside each of these directories, the same set of files will exist:
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nr_hugepages
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nr_hugepages_mempolicy
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nr_overcommit_hugepages
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free_hugepages
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resv_hugepages
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surplus_hugepages
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which function as described above for the default huge page-sized case.
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Interaction of Task Memory Policy with Huge Page Allocation/Freeing
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===================================================================
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Whether huge pages are allocated and freed via the /proc interface or
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the /sysfs interface using the nr_hugepages_mempolicy attribute, the NUMA
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nodes from which huge pages are allocated or freed are controlled by the
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NUMA memory policy of the task that modifies the nr_hugepages_mempolicy
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sysctl or attribute. When the nr_hugepages attribute is used, mempolicy
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is ignored.
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The recommended method to allocate or free huge pages to/from the kernel
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huge page pool, using the nr_hugepages example above, is:
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numactl --interleave <node-list> echo 20 \
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>/proc/sys/vm/nr_hugepages_mempolicy
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or, more succinctly:
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numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy
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This will allocate or free abs(20 - nr_hugepages) to or from the nodes
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specified in <node-list>, depending on whether number of persistent huge pages
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is initially less than or greater than 20, respectively. No huge pages will be
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allocated nor freed on any node not included in the specified <node-list>.
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When adjusting the persistent hugepage count via nr_hugepages_mempolicy, any
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memory policy mode--bind, preferred, local or interleave--may be used. The
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resulting effect on persistent huge page allocation is as follows:
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1) Regardless of mempolicy mode [see Documentation/vm/numa_memory_policy.txt],
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persistent huge pages will be distributed across the node or nodes
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specified in the mempolicy as if "interleave" had been specified.
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However, if a node in the policy does not contain sufficient contiguous
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memory for a huge page, the allocation will not "fallback" to the nearest
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neighbor node with sufficient contiguous memory. To do this would cause
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undesirable imbalance in the distribution of the huge page pool, or
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possibly, allocation of persistent huge pages on nodes not allowed by
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the task's memory policy.
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2) One or more nodes may be specified with the bind or interleave policy.
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If more than one node is specified with the preferred policy, only the
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lowest numeric id will be used. Local policy will select the node where
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the task is running at the time the nodes_allowed mask is constructed.
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For local policy to be deterministic, the task must be bound to a cpu or
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cpus in a single node. Otherwise, the task could be migrated to some
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other node at any time after launch and the resulting node will be
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indeterminate. Thus, local policy is not very useful for this purpose.
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Any of the other mempolicy modes may be used to specify a single node.
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3) The nodes allowed mask will be derived from any non-default task mempolicy,
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whether this policy was set explicitly by the task itself or one of its
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ancestors, such as numactl. This means that if the task is invoked from a
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shell with non-default policy, that policy will be used. One can specify a
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node list of "all" with numactl --interleave or --membind [-m] to achieve
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interleaving over all nodes in the system or cpuset.
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4) Any task mempolicy specifed--e.g., using numactl--will be constrained by
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the resource limits of any cpuset in which the task runs. Thus, there will
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be no way for a task with non-default policy running in a cpuset with a
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subset of the system nodes to allocate huge pages outside the cpuset
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without first moving to a cpuset that contains all of the desired nodes.
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5) Boot-time huge page allocation attempts to distribute the requested number
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of huge pages over all on-lines nodes with memory.
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Per Node Hugepages Attributes
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=============================
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A subset of the contents of the root huge page control directory in sysfs,
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described above, will be replicated under each the system device of each
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NUMA node with memory in:
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/sys/devices/system/node/node[0-9]*/hugepages/
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Under this directory, the subdirectory for each supported huge page size
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contains the following attribute files:
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nr_hugepages
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free_hugepages
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surplus_hugepages
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The free_' and surplus_' attribute files are read-only. They return the number
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of free and surplus [overcommitted] huge pages, respectively, on the parent
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node.
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The nr_hugepages attribute returns the total number of huge pages on the
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specified node. When this attribute is written, the number of persistent huge
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pages on the parent node will be adjusted to the specified value, if sufficient
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resources exist, regardless of the task's mempolicy or cpuset constraints.
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Note that the number of overcommit and reserve pages remain global quantities,
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as we don't know until fault time, when the faulting task's mempolicy is
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applied, from which node the huge page allocation will be attempted.
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Using Huge Pages
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================
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If the user applications are going to request huge pages using mmap system
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call, then it is required that system administrator mount a file system of
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type hugetlbfs:
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mount -t hugetlbfs \
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-o uid=<value>,gid=<value>,mode=<value>,pagesize=<value>,size=<value>,\
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min_size=<value>,nr_inodes=<value> none /mnt/huge
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This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
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/mnt/huge. Any files created on /mnt/huge uses huge pages. The uid and gid
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options sets the owner and group of the root of the file system. By default
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the uid and gid of the current process are taken. The mode option sets the
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mode of root of file system to value & 01777. This value is given in octal.
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By default the value 0755 is picked. If the paltform supports multiple huge
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page sizes, the pagesize option can be used to specify the huge page size and
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associated pool. pagesize is specified in bytes. If pagesize is not specified
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the paltform's default huge page size and associated pool will be used. The
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size option sets the maximum value of memory (huge pages) allowed for that
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filesystem (/mnt/huge). The size option can be specified in bytes, or as a
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percentage of the specified huge page pool (nr_hugepages). The size is
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rounded down to HPAGE_SIZE boundary. The min_size option sets the minimum
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value of memory (huge pages) allowed for the filesystem. min_size can be
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specified in the same way as size, either bytes or a percentage of the
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huge page pool. At mount time, the number of huge pages specified by
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min_size are reserved for use by the filesystem. If there are not enough
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free huge pages available, the mount will fail. As huge pages are allocated
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to the filesystem and freed, the reserve count is adjusted so that the sum
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of allocated and reserved huge pages is always at least min_size. The option
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nr_inodes sets the maximum number of inodes that /mnt/huge can use. If the
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size, min_size or nr_inodes option is not provided on command line then
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no limits are set. For pagesize, size, min_size and nr_inodes options, you
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can use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo. For example, size=2K
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has the same meaning as size=2048.
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While read system calls are supported on files that reside on hugetlb
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file systems, write system calls are not.
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Regular chown, chgrp, and chmod commands (with right permissions) could be
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used to change the file attributes on hugetlbfs.
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Also, it is important to note that no such mount command is required if
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applications are going to use only shmat/shmget system calls or mmap with
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MAP_HUGETLB. For an example of how to use mmap with MAP_HUGETLB see map_hugetlb
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below.
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Users who wish to use hugetlb memory via shared memory segment should be a
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member of a supplementary group and system admin needs to configure that gid
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into /proc/sys/vm/hugetlb_shm_group. It is possible for same or different
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applications to use any combination of mmaps and shm* calls, though the mount of
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filesystem will be required for using mmap calls without MAP_HUGETLB.
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Syscalls that operate on memory backed by hugetlb pages only have their lengths
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aligned to the native page size of the processor; they will normally fail with
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errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
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not hugepage aligned. For example, munmap(2) will fail if memory is backed by
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a hugetlb page and the length is smaller than the hugepage size.
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Examples
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========
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1) map_hugetlb: see tools/testing/selftests/vm/map_hugetlb.c
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2) hugepage-shm: see tools/testing/selftests/vm/hugepage-shm.c
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3) hugepage-mmap: see tools/testing/selftests/vm/hugepage-mmap.c
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4) The libhugetlbfs (https://github.com/libhugetlbfs/libhugetlbfs) library
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provides a wide range of userspace tools to help with huge page usability,
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environment setup, and control.
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Kernel development regression testing
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=====================================
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The most complete set of hugetlb tests are in the libhugetlbfs repository.
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If you modify any hugetlb related code, use the libhugetlbfs test suite
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to check for regressions. In addition, if you add any new hugetlb
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functionality, please add appropriate tests to libhugetlbfs.
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