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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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mbind(2) System Calls Manual mbind(2)
mbind - set memory policy for a memory range
NUMA (Non-Uniform Memory Access) policy library (libnuma, -lnuma)
#include <numaif.h>
long mbind(unsigned long size, unsigned long maxnode;
void addr[size], unsigned long size, int mode,
const unsigned long nodemask[(maxnode + ULONG_WIDTH - 1)
/ ULONG_WIDTH],
unsigned long maxnode, unsigned int flags);
mbind() sets the NUMA memory policy, which consists of a policy
mode and zero or more nodes, for the memory range starting with
addr and continuing for size bytes. The memory policy defines
from which node memory is allocated.
If the memory range specified by the addr and size arguments
includes an "anonymous" region of memory—that is a region of
memory created using the mmap(2) system call with the
MAP_ANONYMOUS—or a memory-mapped file, mapped using the mmap(2)
system call with the MAP_PRIVATE flag, pages will be allocated
only according to the specified policy when the application writes
(stores) to the page. For anonymous regions, an initial read
access will use a shared page in the kernel containing all zeros.
For a file mapped with MAP_PRIVATE, an initial read access will
allocate pages according to the memory policy of the thread that
causes the page to be allocated. This may not be the thread that
called mbind().
The specified policy will be ignored for any MAP_SHARED mappings
in the specified memory range. Rather the pages will be allocated
according to the memory policy of the thread that caused the page
to be allocated. Again, this may not be the thread that called
mbind().
If the specified memory range includes a shared memory region
created using the shmget(2) system call and attached using the
shmat(2) system call, pages allocated for the anonymous or shared
memory region will be allocated according to the policy specified,
regardless of which process attached to the shared memory segment
causes the allocation. If, however, the shared memory region was
created with the SHM_HUGETLB flag, the huge pages will be
allocated according to the policy specified only if the page
allocation is caused by the process that calls mbind() for that
region.
By default, mbind() has an effect only for new allocations; if the
pages inside the range have been already touched before setting
the policy, then the policy has no effect. This default behavior
may be overridden by the MPOL_MF_MOVE and MPOL_MF_MOVE_ALL flags
described below.
The mode argument must specify one of MPOL_DEFAULT, MPOL_BIND,
MPOL_INTERLEAVE, MPOL_WEIGHTED_INTERLEAVE, MPOL_PREFERRED,
MPOL_PREFERRED_MANY, or MPOL_LOCAL (which are described in detail
below). All policy modes except MPOL_DEFAULT require the caller
to specify the node or nodes to which the mode applies, via the
nodemask argument.
The mode argument may also include an optional mode flag. The
supported mode flags are:
MPOL_F_NUMA_BALANCING (since Linux 5.15)
When mode is MPOL_BIND, enable the kernel NUMA balancing
for the task if it is supported by the kernel. If the flag
isn't supported by the kernel, or is used with mode other
than MPOL_BIND, -1 is returned and errno is set to EINVAL.
MPOL_F_STATIC_NODES (since Linux-2.6.26)
A nonempty nodemask specifies physical node IDs. Linux
does not remap the nodemask when the thread moves to a
different cpuset context, nor when the set of nodes allowed
by the thread's current cpuset context changes.
MPOL_F_RELATIVE_NODES (since Linux-2.6.26)
A nonempty nodemask specifies node IDs that are relative to
the set of node IDs allowed by the thread's current cpuset.
nodemask points to a bit mask of nodes containing up to maxnode
bits. The bit mask size is rounded to the next multiple of
sizeof(unsigned long), but the kernel will use bits only up to
maxnode. A NULL value of nodemask or a maxnode value of zero
specifies the empty set of nodes. If the value of maxnode is
zero, the nodemask argument is ignored. Where a nodemask is
required, it must contain at least one node that is on-line,
allowed by the thread's current cpuset context (unless the
MPOL_F_STATIC_NODES mode flag is specified), and contains memory.
The mode argument must include one of the following values:
MPOL_DEFAULT
This mode requests that any nondefault policy be removed,
restoring default behavior. When applied to a range of
memory via mbind(), this means to use the thread memory
policy, which may have been set with set_mempolicy(2). If
the mode of the thread memory policy is also MPOL_DEFAULT,
the system-wide default policy will be used. The system-
wide default policy allocates pages on the node of the CPU
that triggers the allocation. For MPOL_DEFAULT, the
nodemask and maxnode arguments must be specify the empty
set of nodes.
MPOL_BIND
This mode specifies a strict policy that restricts memory
allocation to the nodes specified in nodemask. If nodemask
specifies more than one node, page allocations will come
from the node with sufficient free memory that is closest
to the node where the allocation takes place. Pages will
not be allocated from any node not specified in the IR
nodemask . (Before Linux 2.6.26, page allocations came
from the node with the lowest numeric node ID first, until
that node contained no free memory. Allocations then came
from the node with the next highest node ID specified in
nodemask and so forth, until none of the specified nodes
contained free memory.)
MPOL_INTERLEAVE
This mode specifies that page allocations be interleaved
across the set of nodes specified in nodemask. This
optimizes for bandwidth instead of latency by spreading out
pages and memory accesses to those pages across multiple
nodes. To be effective the memory area should be fairly
large, at least 1 MB or bigger with a fairly uniform access
pattern. Accesses to a single page of the area will still
be limited to the memory bandwidth of a single node.
MPOL_WEIGHTED_INTERLEAVE (since Linux 6.9)
This mode interleaves page allocations across the nodes
specified in nodemask according to the weights in
/sys/kernel/mm/mempolicy/weighted_interleave. For example,
if bits 0, 2, and 5 are set in nodemask, and the contents
of /sys/kernel/mm/mempolicy/weighted_interleave/node0,
/sys/.../node2, and /sys/.../node5 are 4, 7, and 9,
respectively, then pages in this region will be allocated
on nodes 0, 2, and 5 in a 4:7:9 ratio.
MPOL_PREFERRED
This mode sets the preferred node for allocation. The
kernel will try to allocate pages from this node first and
fall back to other nodes if the preferred nodes is low on
free memory. If nodemask specifies more than one node ID,
the first node in the mask will be selected as the
preferred node. If the nodemask and maxnode arguments
specify the empty set, then the memory is allocated on the
node of the CPU that triggered the allocation.
MPOL_PREFERRED_MANY (since Linux 5.15)
Specifies a set of nodes for allocation; see
set_mempolicy(2)
MPOL_LOCAL (since Linux 3.8)
This mode specifies "local allocation"; the memory is
allocated on the node of the CPU that triggered the
allocation (the "local node"). The nodemask and maxnode
arguments must specify the empty set. If the "local node"
is low on free memory, the kernel will try to allocate
memory from other nodes. The kernel will allocate memory
from the "local node" whenever memory for this node is
available. If the "local node" is not allowed by the
thread's current cpuset context, the kernel will try to
allocate memory from other nodes. The kernel will allocate
memory from the "local node" whenever it becomes allowed by
the thread's current cpuset context. By contrast,
MPOL_DEFAULT reverts to the memory policy of the thread
(which may be set via set_mempolicy(2)); that policy may be
something other than "local allocation".
If MPOL_MF_STRICT is passed in flags and mode is not MPOL_DEFAULT,
then the call fails with the error EIO if the existing pages in
the memory range don't follow the policy.
If MPOL_MF_MOVE is specified in flags, then the kernel will
attempt to move all the existing pages in the memory range so that
they follow the policy. Pages that are shared with other
processes will not be moved. If MPOL_MF_STRICT is also specified,
then the call fails with the error EIO if some pages could not be
moved. If the MPOL_INTERLEAVE policy was specified, pages already
residing on the specified nodes will not be moved such that they
are interleaved.
If MPOL_MF_MOVE_ALL is passed in flags, then the kernel will
attempt to move all existing pages in the memory range regardless
of whether other processes use the pages. The calling thread must
be privileged (CAP_SYS_NICE) to use this flag. If MPOL_MF_STRICT
is also specified, then the call fails with the error EIO if some
pages could not be moved. If the MPOL_INTERLEAVE policy was
specified, pages already residing on the specified nodes will not
be moved such that they are interleaved.
On success, mbind() returns 0; on error, -1 is returned and errno
is set to indicate the error.
EFAULT Part or all of the memory range specified by nodemask and
maxnode points outside your accessible address space. Or,
there was an unmapped hole in the specified memory range
specified by addr and size.
EINVAL An invalid value was specified for flags or mode; or addr +
size was less than addr; or addr is not a multiple of the
system page size. Or, mode is MPOL_DEFAULT and nodemask
specified a nonempty set; or mode is MPOL_BIND or
MPOL_INTERLEAVE and nodemask is empty. Or, maxnode exceeds
a kernel-imposed limit. Or, nodemask specifies one or more
node IDs that are greater than the maximum supported node
ID. Or, none of the node IDs specified by nodemask are on-
line and allowed by the thread's current cpuset context, or
none of the specified nodes contain memory. Or, the mode
argument specified both MPOL_F_STATIC_NODES and
MPOL_F_RELATIVE_NODES.
EIO MPOL_MF_STRICT was specified and an existing page was
already on a node that does not follow the policy; or
MPOL_MF_MOVE or MPOL_MF_MOVE_ALL was specified and the
kernel was unable to move all existing pages in the range.
ENOMEM Insufficient kernel memory was available.
EPERM The flags argument included the MPOL_MF_MOVE_ALL flag and
the caller does not have the CAP_SYS_NICE privilege.
Linux.
Linux 2.6.7.
Support for huge page policy was added with Linux 2.6.16. For
interleave policy to be effective on huge page mappings the
policied memory needs to be tens of megabytes or larger.
Before Linux 5.7. MPOL_MF_STRICT was ignored on huge page
mappings.
MPOL_MF_MOVE and MPOL_MF_MOVE_ALL are available only on Linux
2.6.16 and later.
For information on library support, see numa(7).
NUMA policy is not supported on a memory-mapped file range that
was mapped with the MAP_SHARED flag.
The MPOL_DEFAULT mode can have different effects for mbind() and
set_mempolicy(2). When MPOL_DEFAULT is specified for
set_mempolicy(2), the thread's memory policy reverts to the system
default policy or local allocation. When MPOL_DEFAULT is
specified for a range of memory using mbind(), any pages
subsequently allocated for that range will use the thread's memory
policy, as set by set_mempolicy(2). This effectively removes the
explicit policy from the specified range, "falling back" to a
possibly nondefault policy. To select explicit "local allocation"
for a memory range, specify a mode of MPOL_LOCAL or MPOL_PREFERRED
with an empty set of nodes. This method will work for
set_mempolicy(2), as well.
get_mempolicy(2), getcpu(2), mmap(2), set_mempolicy(2), shmat(2),
shmget(2), numa(3), cpuset(7), numa(7), numactl(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 mbind(2)
Pages that refer to this page: getcpu(2), get_mempolicy(2), migrate_pages(2), move_pages(2), set_mempolicy(2), swapon(2), syscalls(2), numa(3), proc_sys_vm(5), capabilities(7), cpuset(7), numa(7), migratepages(8), numactl(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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