mbind(2)
NAME
mbind - set memory policy for a memory range
SYNOPSIS
#include <numaif.h>
long mbind(void *addr, unsigned long len, int mode,
const unsigned long *nodemask, unsigned long maxnode,
unsigned flags);
Link with -lnuma.
DESCRIPTION
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 len bytes. The memory policy defines from which node
memory is allocated.
If the memory range specified by the addr and len 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_PREFERRED, 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_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: The
MPOL_DEFAULT
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 the lowest numeric node ID first, until that node
contains no free memory. Allocations will then come from the
node with the next highest node ID specified in nodemask and so
forth, until none of the specified nodes contain free memory.
Pages will not be allocated from any node not specified in the
nodemask.
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 1MB 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_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_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 will fail 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 will fail with the
error EIO if some pages could not be moved.
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 will fail with the error EIO if some pages could not be
moved.
RETURN VALUE
On success, mbind() returns 0; on error, -1 is returned and errno is
set to indicate the error.
ERRORS
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 len.
EINVAL An invalid value was specified for flags or mode; or addr + len
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.
VERSIONS
The mbind() system call was added to the Linux kernel in version 2.6.7.
CONFORMING TO
This system call is Linux-specific.
NOTES
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.
Support for huge page policy was added with 2.6.16. For interleave
policy to be effective on huge page mappings the policied memory needs
to be tens of megabytes or larger.
MPOL_MF_STRICT is ignored on huge page mappings.
MPOL_MF_MOVE and MPOL_MF_MOVE_ALL are available only on Linux 2.6.16
and later.
SEE ALSO
get_mempolicy(2), getcpu(2), mmap(2), set_mempolicy(2), shmat(2),
shmget(2), numa(3), cpuset(7), numa(7), numactl(8)
COLOPHON
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latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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