path: root/Documentation/vm/hugetlbpage.txt

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 kernel built with huge page support should show the number of configured
huge pages in the system by running the "cat /proc/meminfo" command.

/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 huge page 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:

.....
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 configured hugetlb
pages in the kernel.  Super user can dynamically request more (or free some
pre-configured) huge pages.
The allocation (or deallocation) of hugetlb pages is possible only if there are
enough physically contiguous free pages in system (freeing of huge pages 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.

The administrator can preallocate 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 for preallocating huge pages as
memory has not yet become fragmented.

Some platforms support multiple huge page sizes.  To preallocate 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.

/proc/sys/vm/nr_hugepages indicates the current number of configured [default
size] hugetlb pages in the kernel.  Super user can dynamically request more
(or free some pre-configured) huge pages.

Use the following command to dynamically allocate/deallocate default sized
huge pages:

	echo 20 > /proc/sys/vm/nr_hugepages

This command will try to configure 20 default sized huge pages in the system.
On a NUMA platform, the kernel will attempt to distribute the huge page pool
over the all on-line nodes.  These huge pages, allocated when nr_hugepages
is increased, are called "persistent huge pages".

The success or failure of huge page allocation depends on the amount of
physically contiguous memory that is preset 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 request 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 "surplus"
huge pages from the buddy allocator, when the normal pool is exhausted. As
these surplus huge pages go out of use, they are freed back to the buddy
allocator.

When increasing the huge page pool size via nr_hugepages, any 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 huge page pool size.

The administrator may shrink the pool of preallocated 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 on-line nodes.  Any free huge pages on the selected nodes will
be freed back to the buddy allocator.

Caveat: Shrinking the pool via nr_hugepages such that it becomes less
than the number of huge pages 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 huge page pools at run-time available, much of
the huge page userspace interface has been duplicated in sysfs. The above
information applies to the default huge page size which will be
controlled by the /proc interfaces 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_overcommit_hugepages
	free_hugepages
	resv_hugepages
	surplus_hugepages

which function as described above for the default huge page-sized case.

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.

*******************************************************************

/*
 * Example of using huge page 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 huge pages.
 *
 * For the ia64 architecture, the Linux kernel reserves Region number 4 for
 * huge pages.  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
 */
#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 huge page 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 huge pages.
 * 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.
 */
#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;
}