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_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_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.

VERSIONS

       The set_mempolicy() system call was added in Linux 2.6.7.

STANDARDS

       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).