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, or MPOL_PREFERRED. All modes except MPOL_DEFAULT require the caller to specify via the nodemask argument one or more nodes.

The mode argument may also include an optional mode flag. The supported mode flags are:

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 MPOL_DEFAULT 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.

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 discussed above).

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

VERSIONS

The set_mempolicy(), system call was added to the Linux kernel in version 2.6.7.

CONFORMING TO

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).

SEE ALSO

get_mempolicy(2), getcpu(2), mbind(2), mmap(2), numa(3), cpuset(7), numa(7), numactl(8)