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 allocated.
       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 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, 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 nodemask
       argument.

       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 thread moves to a different cpuset
              context, nor when the set of nodes allowed by the thread'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 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 allocation.  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
              allocation to the nodes specified in nodemask.  If nodemask
              specifies 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 specified 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 kernel 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 memory 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 maxnode
              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 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 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 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_LOCAL or 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.

       Before Linux 5.7.  MPOL_MF_STRICT was 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 5.10 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                              2020-06-09                           MBIND(2)