DESCRIPTION

mbind() sets the NUMA memory policy, which consists of a policy mode and zero or more nodes, for the memory range starting with start and continuing for len bytes. The memory of a NUMA machine is divided into multiple nodes. The memory policy defines from which node memory is allocated.

If the memory range specified by the start and len arguments includes an "anonymous" region of memory—that is a region of memory created using the mmap(2) system call with the MAP_ANONYMOUS—or a memory mapped file, mapped using the mmap(2) system call with the MAP_PRIVATE flag, pages will only be allocated according to the specified policy when the application writes [stores] to the page. For anonymous regions, an initial read access will use a shared page in the kernel containing all zeros. For a file mapped with MAP_PRIVATE, an initial read access will allocate pages according to the process policy of the process that causes the page to be allocated. This may not be the process that called mbind().

The specified policy will be ignored for any MAP_SHARED mappings in the specified memory range. Rather the pages will be allocated according to the process policy of the process that caused the page to be allocated. Again, this may not be the process that called mbind().

If the specified memory range includes a shared memory region created using the shmget(2) system call and attached using the shmat(2) system call, pages allocated for the anonymous or shared memory region will be allocated according to the policy specified, regardless which process attached to the shared memory segment causes the allocation. If, however, the shared memory region was created with the SHM_HUGETLB flag, the huge pages will be allocated according to the policy specified only if the page allocation is caused by the task that calls mbind() for that region.

By default, mbind() only has an effect for new allocations; if the pages inside the range have been already touched before setting the policy, then the policy has no effect. This default behavior may be overridden by the MPOL_MF_MOVE and MPOL_MF_MOVE_ALL flags described below.

The mode argument must specify one of MPOL_DEFAULT, MPOL_BIND, MPOL_INTERLEAVE or MPOL_PREFERRED. All policy modes except MPOL_DEFAULT require the caller to specify via the nodemask parameter, the node or nodes to which the mode applies.

nodemask points to a bitmask of nodes containing 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 specifies that the default policy should be used. When applied to a range of memory via mbind(), this means to use the process policy, which may have been set with set_mempolicy(2). If the mode of the process policy is also MPOL_DEFAULT, the system-wide default policy will be used. The system-wide default policy will allocate pages on the node of the CPU that triggers the allocation. For MPOL_DEFAULT, the nodemask and maxnode arguments must be specify the empty set of nodes.

The MPOL_BIND mode specifies 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.

The MPOL_INTERLEAVE mode specifies that page allocations be interleaved across the set of nodes specified in nodemask. This optimizes for bandwidth instead of latency by spreading out pages and memory accesses to those pages across multiple nodes. To be effective the memory area should be fairly large, at least 1MB or bigger with a fairly uniform access pattern. Accesses to a single page of the area 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 other nodes if the preferred nodes 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. This is the only way to specify "local allocation" for a range of memory via mbind().

If MPOL_MF_STRICT is passed in flags and policy is not MPOL_DEFAULT, then the call will fail with the error EIO if the existing pages in the memory range don't follow the policy.

If MPOL_MF_MOVE is specified in flags, then the kernel will attempt to move all the existing pages in the memory range so that they follow the policy. Pages that are shared with other processes will not be moved. If MPOL_MF_STRICT is also specified, then the call will fail with the error EIO if some pages could not be moved.

If MPOL_MF_MOVE_ALL is passed in flags, then the kernel will attempt to move all existing pages in the memory range regardless of whether other processes use the pages. The calling process must be privileged (CAP_SYS_NICE) to use this flag. If MPOL_MF_STRICT is also specified, then the call will fail with the error EIO if some pages could not be moved.

RETURN VALUE

On success, mbind() 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

NUMA policy is not supported on a memory mapped file range that was mapped with the MAP_SHARED flag.

MPOL_MF_STRICT is ignored on huge page mappings.

The MPOL_DEFAULT, mode has different effects for mbind() and set_mempolicy(2). When MPOL_DEFAULT is specified for a range of memory using mbind(), any pages subsequently allocated for that range will use the process's policy, as set by set_mempolicy(2). This effectively removes the explicit policy from the specified range. To select "local allocation" for a memory range, specify a mode of MPOL_PREFERRED with an empty set of nodes. This method will work for set_mempolicy(2), as well.

SEE ALSO

get_mempolicy(2), mmap(2), set_mempolicy(2), shmat(2), shmget(2), numa(3), numactl(8)