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MBIND(2)                   Linux Programmer's Manual                  MBIND(2)



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 speci-
       fied 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 non-empty 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 non-empty nodemask specifies node ids that are relative to the set of node ids allowed   by  the  pro-
              cess'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 non-default  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  node-
       mask.   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 mem-
       ory 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 maxn-
       ode  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 policy 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 mem-
       ory 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 non-empty 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  con-
              text,  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 speci-
       fied 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 non-default 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 map-
       pings 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-15                          MBIND(2)