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



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

SYNOPSIS
       #include <numaif.h>

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

       Link with -lnuma.

DESCRIPTION
       set_mempolicy()  sets  the  NUMA  memory  policy of the calling process, 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 process.

       This  system  call  defines  the  default  policy for the process.  The process 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  process
       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 process and of memory mapped files mapped using the  mmap(2)
       call  with  the  MAP_SHARED flag, regardless of the access type.  The policy is only applied when a new page is allocated
       for the process.  For anonymous memory this is when the page is first touched by the application.

       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 does 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 only use bits up to maxnode.  A NULL value of nodemask or a  maxn-
       ode 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  MPOL_DEFAULT  mode  specifies that any nondefault process memory policy be removed, so that the memory policy "falls
       back" to the system default policy.  The system default policy is "local allocation"-- i.e., 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 mem-
       ory, 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 node-
       mask  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 opti-
       mizes  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  process memory policy is preserved across an execve(2), and is inherited by child processes 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
       Process 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 process 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 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-08-15                                           SET_MEMPOLICY(2)