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NAME

       set_mempolicy - set default NUMA memory policy for a thread and its children

SYNOPSIS

       #include <numaif.h>

       long set_mempolicy(int mode, const unsigned long *nodemask,
                          unsigned long maxnode);

       Link with -lnuma.

DESCRIPTION

       set_mempolicy()  sets  the  NUMA  memory policy of the calling thread, which consists of a
       policy mode and zero or more nodes, to the values specified  by  the  mode,  nodemask  and
       maxnode arguments.

       A NUMA machine has different memory controllers with different distances to specific CPUs.
       The memory policy defines from which node memory is allocated for the thread.

       This system call defines the default policy for the thread.   The  thread  policy  governs
       allocation  of pages in the process's address space outside of memory ranges controlled by
       a more specific  policy  set  by  mbind(2).   The  thread  default  policy  also  controls
       allocation  of  any  pages  for memory-mapped files mapped using the mmap(2) call with the
       MAP_PRIVATE flag and that are only read (loaded) from by the thread and  of  memory-mapped
       files  mapped  using  the  mmap(2) call with the MAP_SHARED flag, regardless of the access
       type.  The policy is applied only when a new  page  is  allocated  for  the  thread.   For
       anonymous memory this is when the page is first touched by the thread.

       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  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 will 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 node IDs that contains 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
       specified), and contains memory.  If the MPOL_F_STATIC_NODES is set in mode and a required
       nodemask contains no nodes that are allowed by the process's current cpuset  context,  the
       memory  policy  reverts  to  local  allocation.   This effectively overrides the specified
       policy until the process's cpuset context includes one or more of the nodes  specified  by
       nodemask.

       The mode argument must include one of the following values:

       MPOL_DEFAULT
              This  mode  specifies  that any nondefault thread memory policy be removed, so that
              the memory policy "falls back" to the system default policy.   The  system  default
              policy  is  "local allocation"—that is, allocate memory on the node of the CPU that
              triggered the allocation.  nodemask must be specified as NULL.  If the "local node"
              contains  no  free  memory, the system will attempt to allocate memory from a "near
              by" node.

       MPOL_BIND
              This mode defines 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.

       MPOL_INTERLEAVE
              This mode interleaves page allocations across the nodes specified  in  nodemask  in
              numeric  node  ID  order.   This  optimizes  for  bandwidth  instead  of latency by
              spreading out pages and memory accesses  to  those  pages  across  multiple  nodes.
              However, accesses to a single page 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 "near by" nodes if the preferred node
              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 policy specifies "local allocation" (like
              the system default policy discussed above).

       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 process'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  process's  current  cpuset
              context.

       The  thread  memory  policy  is  preserved  across an execve(2), and is inherited by child
       threads created using fork(2) or clone(2).

RETURN VALUE

       On success, set_mempolicy() 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.

       EINVAL mode is invalid.  Or, mode is MPOL_DEFAULT and nodemask is  nonempty,  or  mode  is
              MPOL_BIND  or  MPOL_INTERLEAVE  and  nodemask is empty.  Or, maxnode specifies more
              than a page worth of bits.  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 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.

       ENOMEM Insufficient kernel memory was available.

VERSIONS

       The set_mempolicy() system call was added to the Linux kernel in version 2.6.7.

CONFORMING TO

       This system call is Linux-specific.

NOTES

       Memory policy is not remembered if the page is swapped out.  When such  a  page  is  paged
       back  in,  it  will  use the policy of the thread or memory range that is in effect at the
       time the page is allocated.

       For information on library support, see numa(7).

SEE ALSO

       get_mempolicy(2), getcpu(2), mbind(2), mmap(2), numa(3), cpuset(7), numa(7), numactl(8)

COLOPHON

       This page is part of release 5.02 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/.