1 2The intent of this file is to give a brief summary of hugetlbpage support in 3the Linux kernel. This support is built on top of multiple page size support 4that is provided by most modern architectures. For example, x86 CPUs normally 5support 4K and 2M (1G if architecturally supported) page sizes, ia64 6architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M, 7256M and ppc64 supports 4K and 16M. A TLB is a cache of virtual-to-physical 8translations. Typically this is a very scarce resource on processor. 9Operating systems try to make best use of limited number of TLB resources. 10This optimization is more critical now as bigger and bigger physical memories 11(several GBs) are more readily available. 12 13Users can use the huge page support in Linux kernel by either using the mmap 14system call or standard SYSV shared memory system calls (shmget, shmat). 15 16First the Linux kernel needs to be built with the CONFIG_HUGETLBFS 17(present under "File systems") and CONFIG_HUGETLB_PAGE (selected 18automatically when CONFIG_HUGETLBFS is selected) configuration 19options. 20 21The /proc/meminfo file provides information about the total number of 22persistent hugetlb pages in the kernel's huge page pool. It also displays 23information about the number of free, reserved and surplus huge pages and the 24default huge page size. The huge page size is needed for generating the 25proper alignment and size of the arguments to system calls that map huge page 26regions. 27 28The output of "cat /proc/meminfo" will include lines like: 29 30..... 31HugePages_Total: vvv 32HugePages_Free: www 33HugePages_Rsvd: xxx 34HugePages_Surp: yyy 35Hugepagesize: zzz kB 36 37where: 38HugePages_Total is the size of the pool of huge pages. 39HugePages_Free is the number of huge pages in the pool that are not yet 40 allocated. 41HugePages_Rsvd is short for "reserved," and is the number of huge pages for 42 which a commitment to allocate from the pool has been made, 43 but no allocation has yet been made. Reserved huge pages 44 guarantee that an application will be able to allocate a 45 huge page from the pool of huge pages at fault time. 46HugePages_Surp is short for "surplus," and is the number of huge pages in 47 the pool above the value in /proc/sys/vm/nr_hugepages. The 48 maximum number of surplus huge pages is controlled by 49 /proc/sys/vm/nr_overcommit_hugepages. 50 51/proc/filesystems should also show a filesystem of type "hugetlbfs" configured 52in the kernel. 53 54/proc/sys/vm/nr_hugepages indicates the current number of "persistent" huge 55pages in the kernel's huge page pool. "Persistent" huge pages will be 56returned to the huge page pool when freed by a task. A user with root 57privileges can dynamically allocate more or free some persistent huge pages 58by increasing or decreasing the value of 'nr_hugepages'. 59 60Pages that are used as huge pages are reserved inside the kernel and cannot 61be used for other purposes. Huge pages cannot be swapped out under 62memory pressure. 63 64Once a number of huge pages have been pre-allocated to the kernel huge page 65pool, a user with appropriate privilege can use either the mmap system call 66or shared memory system calls to use the huge pages. See the discussion of 67Using Huge Pages, below. 68 69The administrator can allocate persistent huge pages on the kernel boot 70command line by specifying the "hugepages=N" parameter, where 'N' = the 71number of huge pages requested. This is the most reliable method of 72allocating huge pages as memory has not yet become fragmented. 73 74Some platforms support multiple huge page sizes. To allocate huge pages 75of a specific size, one must precede the huge pages boot command parameters 76with a huge page size selection parameter "hugepagesz=<size>". <size> must 77be specified in bytes with optional scale suffix [kKmMgG]. The default huge 78page size may be selected with the "default_hugepagesz=<size>" boot parameter. 79 80When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages 81indicates the current number of pre-allocated huge pages of the default size. 82Thus, one can use the following command to dynamically allocate/deallocate 83default sized persistent huge pages: 84 85 echo 20 > /proc/sys/vm/nr_hugepages 86 87This command will try to adjust the number of default sized huge pages in the 88huge page pool to 20, allocating or freeing huge pages, as required. 89 90On a NUMA platform, the kernel will attempt to distribute the huge page pool 91over all the set of allowed nodes specified by the NUMA memory policy of the 92task that modifies nr_hugepages. The default for the allowed nodes--when the 93task has default memory policy--is all on-line nodes with memory. Allowed 94nodes with insufficient available, contiguous memory for a huge page will be 95silently skipped when allocating persistent huge pages. See the discussion 96below of the interaction of task memory policy, cpusets and per node attributes 97with the allocation and freeing of persistent huge pages. 98 99The success or failure of huge page allocation depends on the amount of 100physically contiguous memory that is present in system at the time of the 101allocation attempt. If the kernel is unable to allocate huge pages from 102some nodes in a NUMA system, it will attempt to make up the difference by 103allocating extra pages on other nodes with sufficient available contiguous 104memory, if any. 105 106System administrators may want to put this command in one of the local rc 107init files. This will enable the kernel to allocate huge pages early in 108the boot process when the possibility of getting physical contiguous pages 109is still very high. Administrators can verify the number of huge pages 110actually allocated by checking the sysctl or meminfo. To check the per node 111distribution of huge pages in a NUMA system, use: 112 113 cat /sys/devices/system/node/node*/meminfo | fgrep Huge 114 115/proc/sys/vm/nr_overcommit_hugepages specifies how large the pool of 116huge pages can grow, if more huge pages than /proc/sys/vm/nr_hugepages are 117requested by applications. Writing any non-zero value into this file 118indicates that the hugetlb subsystem is allowed to try to obtain that 119number of "surplus" huge pages from the kernel's normal page pool, when the 120persistent huge page pool is exhausted. As these surplus huge pages become 121unused, they are freed back to the kernel's normal page pool. 122 123When increasing the huge page pool size via nr_hugepages, any existing surplus 124pages will first be promoted to persistent huge pages. Then, additional 125huge pages will be allocated, if necessary and if possible, to fulfill 126the new persistent huge page pool size. 127 128The administrator may shrink the pool of persistent huge pages for 129the default huge page size by setting the nr_hugepages sysctl to a 130smaller value. The kernel will attempt to balance the freeing of huge pages 131across all nodes in the memory policy of the task modifying nr_hugepages. 132Any free huge pages on the selected nodes will be freed back to the kernel's 133normal page pool. 134 135Caveat: Shrinking the persistent huge page pool via nr_hugepages such that 136it becomes less than the number of huge pages in use will convert the balance 137of the in-use huge pages to surplus huge pages. This will occur even if 138the number of surplus pages it would exceed the overcommit value. As long as 139this condition holds--that is, until nr_hugepages+nr_overcommit_hugepages is 140increased sufficiently, or the surplus huge pages go out of use and are freed-- 141no more surplus huge pages will be allowed to be allocated. 142 143With support for multiple huge page pools at run-time available, much of 144the huge page userspace interface in /proc/sys/vm has been duplicated in sysfs. 145The /proc interfaces discussed above have been retained for backwards 146compatibility. The root huge page control directory in sysfs is: 147 148 /sys/kernel/mm/hugepages 149 150For each huge page size supported by the running kernel, a subdirectory 151will exist, of the form: 152 153 hugepages-${size}kB 154 155Inside each of these directories, the same set of files will exist: 156 157 nr_hugepages 158 nr_hugepages_mempolicy 159 nr_overcommit_hugepages 160 free_hugepages 161 resv_hugepages 162 surplus_hugepages 163 164which function as described above for the default huge page-sized case. 165 166 167Interaction of Task Memory Policy with Huge Page Allocation/Freeing 168=================================================================== 169 170Whether huge pages are allocated and freed via the /proc interface or 171the /sysfs interface using the nr_hugepages_mempolicy attribute, the NUMA 172nodes from which huge pages are allocated or freed are controlled by the 173NUMA memory policy of the task that modifies the nr_hugepages_mempolicy 174sysctl or attribute. When the nr_hugepages attribute is used, mempolicy 175is ignored. 176 177The recommended method to allocate or free huge pages to/from the kernel 178huge page pool, using the nr_hugepages example above, is: 179 180 numactl --interleave <node-list> echo 20 \ 181 >/proc/sys/vm/nr_hugepages_mempolicy 182 183or, more succinctly: 184 185 numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy 186 187This will allocate or free abs(20 - nr_hugepages) to or from the nodes 188specified in <node-list>, depending on whether number of persistent huge pages 189is initially less than or greater than 20, respectively. No huge pages will be 190allocated nor freed on any node not included in the specified <node-list>. 191 192When adjusting the persistent hugepage count via nr_hugepages_mempolicy, any 193memory policy mode--bind, preferred, local or interleave--may be used. The 194resulting effect on persistent huge page allocation is as follows: 195 1961) Regardless of mempolicy mode [see Documentation/vm/numa_memory_policy.txt], 197 persistent huge pages will be distributed across the node or nodes 198 specified in the mempolicy as if "interleave" had been specified. 199 However, if a node in the policy does not contain sufficient contiguous 200 memory for a huge page, the allocation will not "fallback" to the nearest 201 neighbor node with sufficient contiguous memory. To do this would cause 202 undesirable imbalance in the distribution of the huge page pool, or 203 possibly, allocation of persistent huge pages on nodes not allowed by 204 the task's memory policy. 205 2062) One or more nodes may be specified with the bind or interleave policy. 207 If more than one node is specified with the preferred policy, only the 208 lowest numeric id will be used. Local policy will select the node where 209 the task is running at the time the nodes_allowed mask is constructed. 210 For local policy to be deterministic, the task must be bound to a cpu or 211 cpus in a single node. Otherwise, the task could be migrated to some 212 other node at any time after launch and the resulting node will be 213 indeterminate. Thus, local policy is not very useful for this purpose. 214 Any of the other mempolicy modes may be used to specify a single node. 215 2163) The nodes allowed mask will be derived from any non-default task mempolicy, 217 whether this policy was set explicitly by the task itself or one of its 218 ancestors, such as numactl. This means that if the task is invoked from a 219 shell with non-default policy, that policy will be used. One can specify a 220 node list of "all" with numactl --interleave or --membind [-m] to achieve 221 interleaving over all nodes in the system or cpuset. 222 2234) Any task mempolicy specifed--e.g., using numactl--will be constrained by 224 the resource limits of any cpuset in which the task runs. Thus, there will 225 be no way for a task with non-default policy running in a cpuset with a 226 subset of the system nodes to allocate huge pages outside the cpuset 227 without first moving to a cpuset that contains all of the desired nodes. 228 2295) Boot-time huge page allocation attempts to distribute the requested number 230 of huge pages over all on-lines nodes with memory. 231 232Per Node Hugepages Attributes 233============================= 234 235A subset of the contents of the root huge page control directory in sysfs, 236described above, will be replicated under each the system device of each 237NUMA node with memory in: 238 239 /sys/devices/system/node/node[0-9]*/hugepages/ 240 241Under this directory, the subdirectory for each supported huge page size 242contains the following attribute files: 243 244 nr_hugepages 245 free_hugepages 246 surplus_hugepages 247 248The free_' and surplus_' attribute files are read-only. They return the number 249of free and surplus [overcommitted] huge pages, respectively, on the parent 250node. 251 252The nr_hugepages attribute returns the total number of huge pages on the 253specified node. When this attribute is written, the number of persistent huge 254pages on the parent node will be adjusted to the specified value, if sufficient 255resources exist, regardless of the task's mempolicy or cpuset constraints. 256 257Note that the number of overcommit and reserve pages remain global quantities, 258as we don't know until fault time, when the faulting task's mempolicy is 259applied, from which node the huge page allocation will be attempted. 260 261 262Using Huge Pages 263================ 264 265If the user applications are going to request huge pages using mmap system 266call, then it is required that system administrator mount a file system of 267type hugetlbfs: 268 269 mount -t hugetlbfs \ 270 -o uid=<value>,gid=<value>,mode=<value>,pagesize=<value>,size=<value>,\ 271 min_size=<value>,nr_inodes=<value> none /mnt/huge 272 273This command mounts a (pseudo) filesystem of type hugetlbfs on the directory 274/mnt/huge. Any files created on /mnt/huge uses huge pages. The uid and gid 275options sets the owner and group of the root of the file system. By default 276the uid and gid of the current process are taken. The mode option sets the 277mode of root of file system to value & 01777. This value is given in octal. 278By default the value 0755 is picked. If the paltform supports multiple huge 279page sizes, the pagesize option can be used to specify the huge page size and 280associated pool. pagesize is specified in bytes. If pagesize is not specified 281the paltform's default huge page size and associated pool will be used. The 282size option sets the maximum value of memory (huge pages) allowed for that 283filesystem (/mnt/huge). The size option can be specified in bytes, or as a 284percentage of the specified huge page pool (nr_hugepages). The size is 285rounded down to HPAGE_SIZE boundary. The min_size option sets the minimum 286value of memory (huge pages) allowed for the filesystem. min_size can be 287specified in the same way as size, either bytes or a percentage of the 288huge page pool. At mount time, the number of huge pages specified by 289min_size are reserved for use by the filesystem. If there are not enough 290free huge pages available, the mount will fail. As huge pages are allocated 291to the filesystem and freed, the reserve count is adjusted so that the sum 292of allocated and reserved huge pages is always at least min_size. The option 293nr_inodes sets the maximum number of inodes that /mnt/huge can use. If the 294size, min_size or nr_inodes option is not provided on command line then 295no limits are set. For pagesize, size, min_size and nr_inodes options, you 296can use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo. For example, size=2K 297has the same meaning as size=2048. 298 299While read system calls are supported on files that reside on hugetlb 300file systems, write system calls are not. 301 302Regular chown, chgrp, and chmod commands (with right permissions) could be 303used to change the file attributes on hugetlbfs. 304 305Also, it is important to note that no such mount command is required if 306applications are going to use only shmat/shmget system calls or mmap with 307MAP_HUGETLB. For an example of how to use mmap with MAP_HUGETLB see map_hugetlb 308below. 309 310Users who wish to use hugetlb memory via shared memory segment should be a 311member of a supplementary group and system admin needs to configure that gid 312into /proc/sys/vm/hugetlb_shm_group. It is possible for same or different 313applications to use any combination of mmaps and shm* calls, though the mount of 314filesystem will be required for using mmap calls without MAP_HUGETLB. 315 316Syscalls that operate on memory backed by hugetlb pages only have their lengths 317aligned to the native page size of the processor; they will normally fail with 318errno set to EINVAL or exclude hugetlb pages that extend beyond the length if 319not hugepage aligned. For example, munmap(2) will fail if memory is backed by 320a hugetlb page and the length is smaller than the hugepage size. 321 322 323Examples 324======== 325 3261) map_hugetlb: see tools/testing/selftests/vm/map_hugetlb.c 327 3282) hugepage-shm: see tools/testing/selftests/vm/hugepage-shm.c 329 3303) hugepage-mmap: see tools/testing/selftests/vm/hugepage-mmap.c 331 3324) The libhugetlbfs (http://libhugetlbfs.sourceforge.net) library provides a 333 wide range of userspace tools to help with huge page usability, environment 334 setup, and control. Furthermore it provides useful test cases that should be 335 used when modifying code to ensure no regressions are introduced. 336