set_mempolicy(2) — Linux manual page

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 5.10 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)

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