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NAME

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

LIBRARY

       NUMA (Non-Uniform Memory Access) policy library (libnuma, -lnuma)

SYNOPSIS

       #include <numaif.h>

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

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_WEIGHTED_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_NUMA_BALANCING (since Linux 5.12)
              When mode is MPOL_BIND, enable the kernel NUMA balancing for  the  task  if  it  is
              supported  by  the  kernel.   If the flag isn't supported by the kernel, or is used
              with mode other than MPOL_BIND, -1 is returned and errno is set to EINVAL.

       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.

       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.

       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_WEIGHTED_INTERLEAVE (since Linux 6.9)
              This mode interleaves page allocations  across  the  nodes  specified  in  nodemask
              according  to  the  weights  in  /sys/kernel/mm/mempolicy/weighted_interleave.  For
              example,  if  bits  0,  2,  and  5  are  set  in  nodemask,  and  the  contents  of
              /sys/kernel/mm/mempolicy/weighted_interleave/node0,       /sys/.../node2,       and
              /sys/.../node5 are 4, 7, and 9, respectively, then pages in  this  region  will  be
              allocated on nodes 0, 2, and 5 in a 4:7:9 ratio.

       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.  Or, the MPOL_F_NUMA_BALANCING isn't
              supported by the kernel, or is used with mode other than MPOL_BIND.

       ENOMEM Insufficient kernel memory was available.

STANDARDS

       Linux.

HISTORY

       Linux 2.6.7.

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)