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NAME

       mbind - Set memory policy for a memory range

SYNOPSIS

       #include <numaif.h>

       int mbind(void *start, 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  start  and
       continuing for len bytes.  The memory of a NUMA machine is divided into
       multiple nodes.  The memory policy defines from which  node  memory  is
       allocated.

       If  the  memory range specified by the start 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 task 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  parameter,
       the node or nodes to which the mode applies.

       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.

       The MPOL_DEFAULT mode specifies that the default policy should be used.
       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  will
       allocate  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  of  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 start + len
              was less than start; or start 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 kernel-imposed limit.
              Or, nodemask specifies one or more node  IDs  that  are  greater
              than  the  maximum  supported node ID, or are not allowed in the
              calling task’s context.  Or, none of the node IDs  specified  by
              nodemask  are  on-line,  or  none of the specified nodes contain
              memory.

       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.

CONFORMING TO

       This system call is Linux-specific.

NOTES

       NUMA policy is not supported on a memory mapped  file  range  that  was
       mapped with the MAP_SHARED flag.

       MPOL_MF_STRICT is ignored on huge page mappings.

       The   MPOL_DEFAULT,   mode   has  different  effects  for  mbind()  and
       set_mempolicy(2).  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.  To select "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.

   Versions and Library Support
       The mbind(), get_mempolicy(2), and set_mempolicy(2) system  calls  were
       added  to the Linux kernel with version 2.6.7.  They are only available
       on kernels compiled with CONFIG_NUMA.

       You can link with -lnuma to get system call definitions.   libnuma  and
       the required <numaif.h> header are available in the numactl package.

       However,  applications  should  not  use  these  system calls directly.
       Instead, the higher level interface provided by the  numa(3)  functions
       in  the  numactl  package  is  recommended.   The  numactl  package  is
       available at ftp://ftp.suse.com/pub/people/ak/numa/.   The  package  is
       also  included in some Linux distributions.  Some distributions include
       the development  library  and  header  in  the  separate  numactl-devel
       package.

       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_MOVE  and  MPOL_MF_MOVE_ALL  are only available on Linux 2.6.16
       and later.

SEE ALSO

       get_mempolicy(2),  mmap(2),  set_mempolicy(2),   shmat(2),   shmget(2),
       numa(3), numactl(8)

COLOPHON

       This  page  is  part of release 2.77 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/.