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 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 spec‐
       ified], 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  band‐
       width 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 alloca‐
       tion.  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 exist‐
       ing  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  call‐
       ing 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 spec‐
              ifies  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 pol‐
       icy, 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.

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 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2014-05-10                                   MBIND(2)