343 строки
17 KiB
Plaintext
343 строки
17 KiB
Plaintext
|
|
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, x86 CPUs normally
|
|
support 4K and 2M (1G if architecturally supported) 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 /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.
|
|
|
|
The output of "cat /proc/meminfo" will include 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 "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 precede 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 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 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 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 specified--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 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>,pagesize=<value>,size=<value>,\
|
|
min_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 & 01777. This value is given in octal.
|
|
By default the value 0755 is picked. If the platform supports multiple huge
|
|
page sizes, the pagesize option can be used to specify the huge page size and
|
|
associated pool. pagesize is specified in bytes. If pagesize is not specified
|
|
the platform's default huge page size and associated pool will be used. The
|
|
size option sets the maximum value of memory (huge pages) allowed for that
|
|
filesystem (/mnt/huge). The size option can be specified in bytes, or as a
|
|
percentage of the specified huge page pool (nr_hugepages). The size is
|
|
rounded down to HPAGE_SIZE boundary. The min_size option sets the minimum
|
|
value of memory (huge pages) allowed for the filesystem. min_size can be
|
|
specified in the same way as size, either bytes or a percentage of the
|
|
huge page pool. At mount time, the number of huge pages specified by
|
|
min_size are reserved for use by the filesystem. If there are not enough
|
|
free huge pages available, the mount will fail. As huge pages are allocated
|
|
to the filesystem and freed, the reserve count is adjusted so that the sum
|
|
of allocated and reserved huge pages is always at least min_size. The option
|
|
nr_inodes sets the maximum number of inodes that /mnt/huge can use. If the
|
|
size, min_size or nr_inodes option is not provided on command line then
|
|
no limits are set. For pagesize, size, min_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
|
|
applications are going to use only shmat/shmget system calls or mmap with
|
|
MAP_HUGETLB. For an example of how to use mmap with MAP_HUGETLB see map_hugetlb
|
|
below.
|
|
|
|
Users who wish to use hugetlb memory 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.
|
|
|
|
Syscalls that operate on memory backed by hugetlb pages only have their lengths
|
|
aligned to the native page size of the processor; they will normally fail with
|
|
errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
|
|
not hugepage aligned. For example, munmap(2) will fail if memory is backed by
|
|
a hugetlb page and the length is smaller than the hugepage size.
|
|
|
|
|
|
Examples
|
|
========
|
|
|
|
1) map_hugetlb: see tools/testing/selftests/vm/map_hugetlb.c
|
|
|
|
2) hugepage-shm: see tools/testing/selftests/vm/hugepage-shm.c
|
|
|
|
3) hugepage-mmap: see tools/testing/selftests/vm/hugepage-mmap.c
|
|
|
|
4) The libhugetlbfs (https://github.com/libhugetlbfs/libhugetlbfs) library
|
|
provides a wide range of userspace tools to help with huge page usability,
|
|
environment setup, and control.
|
|
|
|
Kernel development regression testing
|
|
=====================================
|
|
|
|
The most complete set of hugetlb tests are in the libhugetlbfs repository.
|
|
If you modify any hugetlb related code, use the libhugetlbfs test suite
|
|
to check for regressions. In addition, if you add any new hugetlb
|
|
functionality, please add appropriate tests to libhugetlbfs.
|