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Diffstat (limited to 'Documentation/vm/hugetlbpage.txt')
| -rw-r--r-- | Documentation/vm/hugetlbpage.txt | 474 |
1 files changed, 222 insertions, 252 deletions
diff --git a/Documentation/vm/hugetlbpage.txt b/Documentation/vm/hugetlbpage.txt index ea8714fcc3a..457634c1e03 100644 --- a/Documentation/vm/hugetlbpage.txt +++ b/Documentation/vm/hugetlbpage.txt @@ -11,23 +11,21 @@ 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). +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 kernel built with hugepage support should show the number of configured -hugepages in the system by running the "cat /proc/meminfo" command. +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. -/proc/meminfo also provides information about the total number of hugetlb -pages configured in the kernel. It also displays information about the -number of free hugetlb pages at any time. It also displays information about -the configured hugepage size - this is needed for generating the proper -alignment and size of the arguments to the above system calls. - -The output of "cat /proc/meminfo" will have lines like: +The output of "cat /proc/meminfo" will include lines like: ..... HugePages_Total: vvv @@ -37,88 +35,231 @@ HugePages_Surp: yyy Hugepagesize: zzz kB where: -HugePages_Total is the size of the pool of hugepages. -HugePages_Free is the number of hugepages in the pool that are not yet -allocated. -HugePages_Rsvd is short for "reserved," and is the number of hugepages -for which a commitment to allocate from the pool has been made, but no -allocation has yet been made. It's vaguely analogous to overcommit. -HugePages_Surp is short for "surplus," and is the number of hugepages in -the pool above the value in /proc/sys/vm/nr_hugepages. The maximum -number of surplus hugepages is controlled by -/proc/sys/vm/nr_overcommit_hugepages. +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 configured hugetlb -pages in the kernel. Super user can dynamically request more (or free some -pre-configured) hugepages. -The allocation (or deallocation) of hugetlb pages is possible only if there are -enough physically contiguous free pages in system (freeing of hugepages is -possible only if there are enough hugetlb pages free that can be transferred -back to regular memory pool). - -Pages that are used as hugetlb pages are reserved inside the kernel and cannot -be used for other purposes. - -Once the kernel with Hugetlb page support is built and running, a user can -use either the mmap system call or shared memory system calls to start using -the huge pages. It is required that the system administrator preallocate -enough memory for huge page purposes. - -Use the following command to dynamically allocate/deallocate hugepages: +/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 preceed 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 configure 20 hugepages in the system. The success -or failure of allocation depends on the amount of physically contiguous -memory that is preset in system at this time. System administrators may want -to put this command in one of the local rc init files. This will enable the -kernel to request huge pages early in the boot process (when the possibility -of getting physical contiguous pages is still very high). In either -case, administrators will want to verify the number of hugepages actually -allocated by checking the sysctl or meminfo. - -/proc/sys/vm/nr_overcommit_hugepages indicates how large the pool of -hugepages can grow, if more hugepages than /proc/sys/vm/nr_hugepages are -requested by applications. echo'ing any non-zero value into this file -indicates that the hugetlb subsystem is allowed to try to obtain -hugepages from the buddy allocator, if the normal pool is exhausted. As -these surplus hugepages go out of use, they are freed back to the buddy -allocator. - -Caveat: Shrinking the pool via nr_hugepages such that it becomes less -than the number of hugepages in use will convert the balance to surplus -huge pages even if it would exceed the overcommit value. As long as -this condition holds, however, no more surplus huge pages will be -allowed on the system until one of the two sysctls are increased -sufficiently, or the surplus huge pages go out of use and are freed. - -With support for multiple hugepage pools at run-time available, much of -the hugepage userspace interface has been duplicated in sysfs. The above -information applies to the default hugepage size (which will be -controlled by the proc interfaces for backwards compatibility). The root -hugepage control directory is +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 hugepage size supported by the running kernel, a subdirectory -will exist, of the form +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 hugepage-sized case. +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 hugepages using mmap system +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: @@ -127,7 +268,7 @@ type hugetlbfs: none /mnt/huge This command mounts a (pseudo) filesystem of type hugetlbfs on the directory -/mnt/huge. Any files created on /mnt/huge uses hugepages. The uid and gid +/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. @@ -146,194 +287,23 @@ 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. 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. +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. ******************************************************************* /* - * Example of using hugepage memory in a user application using Sys V shared - * memory system calls. In this example the app is requesting 256MB of - * memory that is backed by huge pages. The application uses the flag - * SHM_HUGETLB in the shmget system call to inform the kernel that it is - * requesting hugepages. - * - * For the ia64 architecture, the Linux kernel reserves Region number 4 for - * hugepages. That means the addresses starting with 0x800000... will need - * to be specified. Specifying a fixed address is not required on ppc64, - * i386 or x86_64. - * - * Note: The default shared memory limit is quite low on many kernels, - * you may need to increase it via: - * - * echo 268435456 > /proc/sys/kernel/shmmax - * - * This will increase the maximum size per shared memory segment to 256MB. - * The other limit that you will hit eventually is shmall which is the - * total amount of shared memory in pages. To set it to 16GB on a system - * with a 4kB pagesize do: - * - * echo 4194304 > /proc/sys/kernel/shmall + * hugepage-shm: see Documentation/vm/hugepage-shm.c */ -#include <stdlib.h> -#include <stdio.h> -#include <sys/types.h> -#include <sys/ipc.h> -#include <sys/shm.h> -#include <sys/mman.h> - -#ifndef SHM_HUGETLB -#define SHM_HUGETLB 04000 -#endif - -#define LENGTH (256UL*1024*1024) - -#define dprintf(x) printf(x) - -/* Only ia64 requires this */ -#ifdef __ia64__ -#define ADDR (void *)(0x8000000000000000UL) -#define SHMAT_FLAGS (SHM_RND) -#else -#define ADDR (void *)(0x0UL) -#define SHMAT_FLAGS (0) -#endif - -int main(void) -{ - int shmid; - unsigned long i; - char *shmaddr; - - if ((shmid = shmget(2, LENGTH, - SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W)) < 0) { - perror("shmget"); - exit(1); - } - printf("shmid: 0x%x\n", shmid); - - shmaddr = shmat(shmid, ADDR, SHMAT_FLAGS); - if (shmaddr == (char *)-1) { - perror("Shared memory attach failure"); - shmctl(shmid, IPC_RMID, NULL); - exit(2); - } - printf("shmaddr: %p\n", shmaddr); - - dprintf("Starting the writes:\n"); - for (i = 0; i < LENGTH; i++) { - shmaddr[i] = (char)(i); - if (!(i % (1024 * 1024))) - dprintf("."); - } - dprintf("\n"); - - dprintf("Starting the Check..."); - for (i = 0; i < LENGTH; i++) - if (shmaddr[i] != (char)i) - printf("\nIndex %lu mismatched\n", i); - dprintf("Done.\n"); - - if (shmdt((const void *)shmaddr) != 0) { - perror("Detach failure"); - shmctl(shmid, IPC_RMID, NULL); - exit(3); - } - - shmctl(shmid, IPC_RMID, NULL); - - return 0; -} ******************************************************************* /* - * Example of using hugepage memory in a user application using the mmap - * system call. Before running this application, make sure that the - * administrator has mounted the hugetlbfs filesystem (on some directory - * like /mnt) using the command mount -t hugetlbfs nodev /mnt. In this - * example, the app is requesting memory of size 256MB that is backed by - * huge pages. - * - * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages. - * That means the addresses starting with 0x800000... will need to be - * specified. Specifying a fixed address is not required on ppc64, i386 - * or x86_64. + * hugepage-mmap: see Documentation/vm/hugepage-mmap.c */ -#include <stdlib.h> -#include <stdio.h> -#include <unistd.h> -#include <sys/mman.h> -#include <fcntl.h> - -#define FILE_NAME "/mnt/hugepagefile" -#define LENGTH (256UL*1024*1024) -#define PROTECTION (PROT_READ | PROT_WRITE) - -/* Only ia64 requires this */ -#ifdef __ia64__ -#define ADDR (void *)(0x8000000000000000UL) -#define FLAGS (MAP_SHARED | MAP_FIXED) -#else -#define ADDR (void *)(0x0UL) -#define FLAGS (MAP_SHARED) -#endif - -void check_bytes(char *addr) -{ - printf("First hex is %x\n", *((unsigned int *)addr)); -} - -void write_bytes(char *addr) -{ - unsigned long i; - - for (i = 0; i < LENGTH; i++) - *(addr + i) = (char)i; -} - -void read_bytes(char *addr) -{ - unsigned long i; - - check_bytes(addr); - for (i = 0; i < LENGTH; i++) - if (*(addr + i) != (char)i) { - printf("Mismatch at %lu\n", i); - break; - } -} - -int main(void) -{ - void *addr; - int fd; - - fd = open(FILE_NAME, O_CREAT | O_RDWR, 0755); - if (fd < 0) { - perror("Open failed"); - exit(1); - } - - addr = mmap(ADDR, LENGTH, PROTECTION, FLAGS, fd, 0); - if (addr == MAP_FAILED) { - perror("mmap"); - unlink(FILE_NAME); - exit(1); - } - - printf("Returned address is %p\n", addr); - check_bytes(addr); - write_bytes(addr); - read_bytes(addr); - - munmap(addr, LENGTH); - close(fd); - unlink(FILE_NAME); - - return 0; -} |