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


       #include <numaif.h>

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

       Link with -lnuma.


       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, or
       MPOL_PREFERRED.  All modes except MPOL_DEFAULT require  the  caller  to  specify  via  the
       nodemask argument one or more nodes.

       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

       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

       The MPOL_DEFAULT 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.

       The MPOL_BIND 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  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 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).

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


       On success, set_mempolicy() returns 0; on error, -1  is  returned  and  errno  is  set  to
       indicate the error.


       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

       ENOMEM Insufficient kernel memory was available.


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


       This system call is Linux-specific.


       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).


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


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