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
       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
       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
       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 5.04 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)