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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  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 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 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 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 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 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  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 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 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 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 3.23 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/.