mbind (2)
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Copyright 2003,2004 Andi Kleen, SuSE Labs. and Copyright 2007 Lee Schermerhorn, Hewlett Packard %%%LICENSE_START(VERBATIM_PROF) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the te...
NAME
mbind - set memory policy for a memory rangeSYNOPSIS
#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 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() 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, 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 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 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 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 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.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 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 available only on Linux 2.6.16 and later.