NAME¶
mbind - set memory policy for a memory range
SYNOPSIS¶
#include <numaif.h>
int mbind(void *addr, unsigned long len, int mode,
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 only
be allocated 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 process policy of the process that causes the page to be allocated. This
may not be the process 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
process policy of the process that caused the page to be allocated. Again,
this may not be the process 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 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() only has an effect 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 or
MPOL_PREFERRED. All policy
modes except
MPOL_DEFAULT require the caller to specify via the
nodemask argument, the node or nodes to which the mode applies.
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 process moves to a
different cpuset context, nor when the set of nodes allowed by the
process'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 process's current
cpuset.
nodemask points to a bitmask 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 only use bits 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 process's current cpuset
context [unless the
MPOL_F_STATIC_NODES mode flag is specified], and
contains memory.
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 process policy, which may have been set
with
set_mempolicy(2). If the mode of the process 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.
The
MPOL_BIND 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.
The
MPOL_INTERLEAVE 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 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. This is the only way to specify "local allocation" for a
range of memory via
mbind().
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 process 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.
- 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 process'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.
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 process's policy reverts to 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 process's 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_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 only available 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¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.