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hugetlb: clean up and update huge pages documentation

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Attempt to clarify huge page administration and usage, and updates the
doucmentation to mention the balancing of huge pages across nodes when
allocating and freeing.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Nishanth Aravamudan <nacc@us.ibm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Adam Litke <agl@us.ibm.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Eric Whitney <eric.whitney@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Lee Schermerhorn 2009-09-21 17:01:24 -07:00 committed by Linus Torvalds
parent 685f345708
commit 41a25e7e67
1 changed files with 86 additions and 45 deletions
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131
Documentation/vm/hugetlbpage.txt
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@ -18,13 +18,13 @@ First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
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 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 hugepage size - this is needed for generating the proper
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:
@ -37,25 +37,27 @@ 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.
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 hugepages is
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).
@ -67,43 +69,82 @@ 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:
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 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.
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".
/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
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 hugepages in use will convert the balance to surplus
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 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
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 hugepage size supported by the running kernel, a subdirectory
For each huge page size supported by the running kernel, a subdirectory
will exist, of the form
hugepages-${size}kB
@ -116,9 +157,9 @@ Inside each of these directories, the same set of files will exist:
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.
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 +168,7 @@ 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.
@ -156,14 +197,14 @@ mount of filesystem will be required for using mmap calls.
*******************************************************************
/*
* Example of using hugepage memory in a user application using Sys V shared
* 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 hugepages.
* requesting huge pages.
*
* For the ia64 architecture, the Linux kernel reserves Region number 4 for
* hugepages. That means the addresses starting with 0x800000... will need
* 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.
*
@ -252,14 +293,14 @@ int main(void)
*******************************************************************
/*
* Example of using hugepage memory in a user application using the mmap
* 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 hugepages.
* 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.