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 5.11 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                              2020-12-21                   SET_MEMPOLICY(2)