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

NAME

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

LIBRARY

       NUMA (Non-Uniform Memory Access) policy library (libnuma, -lnuma)

SYNOPSIS

       #include <numaif.h>

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

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_WEIGHTED_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_NUMA_BALANCING (since Linux 5.12)
When mode is MPOL_BIND, enable the kernel NUMA balancing for the task if it is supported by the
kernel. If the flag isn't supported by the kernel, or is used with mode other than MPOL_BIND, -1
is returned and errno is set to EINVAL.

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.

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.

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_WEIGHTED_INTERLEAVE (since Linux 6.9)
This mode interleaves page allocations across the nodes specified in nodemask according to the
weights in /sys/kernel/mm/mempolicy/weighted_interleave. For example, if bits 0, 2, and 5 are set
in nodemask, and the contents of /sys/kernel/mm/mempolicy/weighted_interleave/node0,
/sys/.../node2, and /sys/.../node5 are 4, 7, and 9, respectively, then pages in this region will
be allocated on nodes 0, 2, and 5 in a 4:7:9 ratio.

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.  Or, the MPOL_F_NUMA_BALANCING isn't supported by
              the kernel, or is used with mode other than MPOL_BIND.

       ENOMEM Insufficient kernel memory was available.

STANDARDS

       Linux.

HISTORY

       Linux 2.6.7.

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).