DESCRIPTION

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 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 process.

This system call defines the default policy for the process. The process policy governs allocation of pages in the process' address space outside of memory ranges controlled by a more specific policy set by mbind(2). The process 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 task and of memory mapped files mapped using the mmap(2) call with the MAP_SHARED flag, regardless of the access type. The policy is only applied 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 nodemask parameter one or more nodes.

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 only use bits 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.

The MPOL_DEFAULT mode is the default and means to allocate memory locally, i.e., 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 memory is allocated on the node of the CPU that triggered the allocation (like MPOL_DEFAULT).

The process memory policy is preserved across an execve(2), and is inherited by child processes 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

CONFORMING TO

This system call is Linux-specific.

NOTES

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.

SEE ALSO

mbind(2), mmap(2), get_mempolicy(2), numactl(8), numa(3)