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hugetlb: update hugetlb documentation for NUMA controls

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Update the kernel huge tlb documentation to describe the numa memory
policy based huge page management.  Additionaly, the patch includes a fair
amount of rework to improve consistency, eliminate duplication and set the
context for documenting the memory policy interaction.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Andi Kleen <andi@firstfloor.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Nishanth Aravamudan <nacc@us.ibm.com>
Cc: Adam Litke <agl@us.ibm.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Eric Whitney <eric.whitney@hp.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
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 2009-12-14 17:58:30 -08:00 committed by Linus Torvalds
parent 9a30523066
commit 267b4c281b
1 changed files with 175 additions and 84 deletions
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259
Documentation/vm/hugetlbpage.txt
View file

@ -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 huge page support should show the number of configured
huge pages 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 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:
The output of "cat /proc/meminfo" will include lines like:
.....
HugePages_Total: vvv
@ -53,59 +51,63 @@ HugePages_Surp is short for "surplus," and is the number of huge pages in
/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).
/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 hugetlb pages are reserved inside the kernel and cannot
be used for other purposes.
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 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.
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 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.
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 preallocate huge pages
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.
/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:
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 default sized huge pages in the system.
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 the all on-line nodes. These huge pages, allocated when nr_hugepages
is increased, are called "persistent huge pages".
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. 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 preset in system at the time of the
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 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
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
@ -113,45 +115,47 @@ distribution of huge pages in a NUMA system, use:
/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.
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 surplus
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 huge page pool size.
the new persistent huge page pool size.
The administrator may shrink the pool of preallocated huge pages for
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 on-line nodes. Any free huge pages on the selected nodes will
be freed back to the buddy allocator.
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 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.
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 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:
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 huge page size supported by the running kernel, a subdirectory
will exist, of the form
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
@ -159,6 +163,101 @@ Inside each of these directories, the same set of files will exist:
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.
Per Node Hugepages Attributes
A subset of the contents of the root huge page control directory in sysfs,
described above, has been replicated under each "node" system device 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 huge pages using mmap system
call, then it is required that system administrator mount a file system of
type hugetlbfs:
@ -206,9 +305,11 @@ map_hugetlb.c.
* 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.
* huge pages. That means that if one requires a fixed address, a huge page
* aligned address starting with 0x800000... will be required. If a fixed
* address is not required, the kernel will select an address in the proper
* range.
* Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
*
* Note: The default shared memory limit is quite low on many kernels,
* you may need to increase it via:
@ -237,14 +338,8 @@ map_hugetlb.c.
#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 ADDR (void *)(0x0UL) /* let kernel choose address */
#define SHMAT_FLAGS (0)
#endif
int main(void)
{
@ -302,10 +397,12 @@ int main(void)
* 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.
* For the ia64 architecture, the Linux kernel reserves Region number 4 for
* huge pages. That means that if one requires a fixed address, a huge page
* aligned address starting with 0x800000... will be required. If a fixed
* address is not required, the kernel will select an address in the proper
* range.
* Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
*/
#include <stdlib.h>
#include <stdio.h>
@ -317,14 +414,8 @@ int main(void)
#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 ADDR (void *)(0x0UL) /* let kernel choose address */
#define FLAGS (MAP_SHARED)
#endif
void check_bytes(char *addr)
{