mbind

MBIND(2)                   Linux Programmer's Manual                  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  allo-
       cated.  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 speci-
       fied 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  node-
       mask argument.

       The  mode  argument  may  also include an optional mode flag.  The sup-
       ported 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  cur-
              rent 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:

       MPOL_DEFAULT
              This 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 alloca-
              tion.  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  allo-
              cation  to  the nodes specified in nodemask.  If nodemask speci-
              fies more than one node, page allocations  will  come  from  the
              node  with  sufficient  free  memory that is closest to the node
              where the allocation takes place.  Pages will not  be  allocated
              from  any node not specified in the IR nodemask .  (Before Linux
              2.6.26, page allocations came from  the  node  with  the  lowest
              numeric node ID first, until that node contained no free memory.
              Allocations then came from the node with the next  highest  node
              ID  specified in nodemask and so forth, until none of the speci-
              fied nodes contained free memory.)

       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 1 MB 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  ker-
              nel  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 mem-
              ory 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  fails 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 fails 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 fails 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  maxn-
              ode 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  node-
              mask 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 speci-
              fied 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 effec-
       tively removes the explicit policy from the specified  range,  "falling
       back" to a possibly nondefault policy.  To select explicit "local allo-
       cation" for a memory range, specify a mode of MPOL_LOCAL  or  MPOL_PRE-
       FERRED  with an empty set of nodes.  This method will work for set_mem-
       policy(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
       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          MBIND(2)
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