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man: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 4.16 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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