SET_MEMPOLICY(2) Linux Programmer's Manual SET_MEMPOLICY(2)
set_mempolicy - set default NUMA memory policy for a process and its children
int set_mempolicy(int mode, unsigned long *nodemask,
unsigned long maxnode);
Link with -lnuma.
set_mempolicy() sets the NUMA memory policy of the calling process, which consists of a
policy mode and zero or more nodes, to the values specified by the mode, nodemask and
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 process.
This system call defines the default policy for the process. The process 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 process default policy also controls alloca-
tion of any pages for memory mapped files mapped using the mmap(2) call with the MAP_PRI-
VATE flag and that are only read [loaded] from by the process 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 process. For anonymous
memory this is when the page is first touched by the application.
The mode argument must specify one of MPOL_DEFAULT, MPOL_BIND, MPOL_INTERLEAVE or
MPOL_PREFERRED. All modes except MPOL_DEFAULT require the caller to specify via the node-
mask argument one or more nodes.
The mode argument may also include an optional mode flag. The supported mode flags are:
MPOL_F_STATIC_NODES (since Linux 2.6.26)
A nonempty nodemask specifies physical node ids. Linux does 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.
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.
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
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 pol-
icy until the process's cpuset context includes one or more of the nodes specified by
The MPOL_DEFAULT mode specifies that any nondefault process memory policy be removed, so
that the memory policy "falls back" to the system default policy. The system default pol-
icy is "local allocation"-- i.e., 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.
The MPOL_BIND 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 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 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
The process memory policy is preserved across an execve(2), and is inherited by child pro-
cesses created using fork(2) or clone(2).
On success, set_mempolicy() returns 0; on error, -1 is returned and errno is set to indi-
cate 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.
ENOMEM Insufficient kernel memory was available.
The set_mempolicy(), system call was added to the Linux kernel in version 2.6.7.
This system call is Linux-specific.
Process 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 process 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 release 3.55 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2008-08-15 SET_MEMPOLICY(2)