set_mempolicy - set default NUMA memory policy for a thread and its children
long set_mempolicy(int mode, const unsigned long *nodemask,
unsigned long maxnode);
Link with -lnuma.
() 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
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
argument must specify one of MPOL_DEFAULT
(which are described in detail below). All modes except
require the caller to specify the node or nodes to which
the mode applies, via the nodemask
argument may also include an optional mode flag
supported mode flags
- 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
- 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.
points to a bit mask of node IDs that contains up to
bits. The bit mask size is rounded to the next multiple of
, but the kernel will use bits only up to
. A NULL value of nodemask
or a maxnode
zero specifies the empty set of nodes. If the value of maxnode
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
mode flag is specified), and contains memory. If
is set in mode
and a required
contains no nodes that are allowed by the process's current
cpuset context, the memory policy reverts to local allocation
effectively overrides the specified policy until the process's cpuset context
includes one or more of the nodes specified by nodemask
argument must include one of the following values:
- 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.
- 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
- 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.
- 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
The thread memory policy is preserved across an execve
(2), and is
inherited by child threads created using fork
(2) or clone
On success, set_mempolicy
() returns 0; on error, -1 is returned and
is set to indicate the error.
- Part of all of the memory range specified by nodemask and
maxnode points outside your accessible address space.
- 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.
- Insufficient kernel memory was available.
() system call was added to the Linux kernel in version
This system call is Linux-specific.
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