|
HTML rendering created 2025-09-06 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. |
|
|
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
|
|
|
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,
MPOL_PREFERRED_MANY, 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_PREFERRED_MANY (since Linux 5.15)
This mode specifies a preference for nodes from which the
kernel will try to allocate from. This differs from
MPOL_PREFERRED in that it accepts a set of nodes versus a
single node. This policy is intended to benefit page
allocations where specific memory types (i.e. non-volatile,
high-bandwidth, or accelerator memory) are of greater
importance than node location.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 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)
|
HTML rendering created 2025-09-06 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. |
|