NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #include <numaif.h>

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

       Link with -lnuma.

DESCRIPTION         top

       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         top

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

ERRORS         top

       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         top

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

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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         top

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

COLOPHON         top

       This page is part of release 4.11 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                            2016-12-12                 SET_MEMPOLICY(2)

Pages that refer to this page: getcpu(2)get_mempolicy(2)mbind(2)migrate_pages(2)move_pages(2)syscalls(2)numa(3)numa_maps(5)cpuset(7)numa(7)migratepages(8)numactl(8)