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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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set_mempolicy(2) System Calls Manual set_mempolicy(2)
set_mempolicy - set default NUMA memory policy for a thread and
its children
NUMA (Non-Uniform Memory Access) policy library (libnuma, -lnuma)
#include <numaif.h>
long set_mempolicy(int mode, const unsigned long *nodemask,
unsigned long maxnode);
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).
On success, set_mempolicy() returns 0; on error, -1 is returned
and errno is set to indicate the error.
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.
Linux.
Linux 2.6.7.
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).
get_mempolicy(2), getcpu(2), mbind(2), mmap(2), numa(3),
cpuset(7), numa(7), numactl(8)
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Linux man-pages 6.9.1 2024-05-02 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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