set_mempolicy

SET_MEMPOLICY(2)           Linux Programmer's Manual          SET_MEMPOLICY(2)

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  dis-
       tances  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 out-
       side 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 sup-
       ported 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 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 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  alloca-
              tion"--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  alloca-
              tion  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 node-
              mask.

       MPOL_INTERLEAVE
              This mode interleaves page allocations across the  nodes  speci-
              fied  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 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  process'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
              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  maxn-
              ode 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 ver-
       sion 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 4.15 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                  SET_MEMPOLICY(2)
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