Manpage of MBIND
MBINDSection: Linux Programmer's Manual (2)
NAMEmbind - set memory policy for a memory range
#include <numaif.h>long mbind(void *addr, unsigned long len, int mode, const unsigned long *nodemask, unsigned long maxnode, unsigned flags);Link with -lnuma.
DESCRIPTIONmbind() sets the NUMA memory policy, which consists of a policy mode and zero or more nodes, for the memory range starting with addrand continuing for lenbytes. The memory policy defines from which node memory is allocated.
If the memory range specified by the addr and lenarguments 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_PRIVATEflag, pages will be allocated only 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 memory policy of the thread that causes the page to be allocated. This may not be the thread that called mbind().
The specified policy will be ignored for any MAP_SHAREDmappings in the specified memory range. Rather the pages will be allocated according to the memory policy of the thread that caused the page to be allocated. Again, this may not be the thread 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 of which process attached to the shared memory segment causes the allocation. If, however, the shared memory region was created with the SHM_HUGETLBflag, the huge pages will be allocated according to the policy specified only if the page allocation is caused by the process that calls mbind() for that region.
By default, mbind() has an effect only 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_MOVEand MPOL_MF_MOVE_ALLflags described below.
The modeargument must specify one of MPOL_DEFAULT, MPOL_BIND, MPOL_INTERLEAVE, MPOL_PREFERRED, or MPOL_LOCAL(which are described in detail below). All policy modes except MPOL_DEFAULTrequire the caller to specify the node or nodes to which the mode applies, via the nodemaskargument.
The modeargument may also include an optional mode flag. The supported mode flagsare:
- MPOL_F_STATIC_NODES (since Linux-2.6.26)
- A nonempty nodemaskspecifies physical node IDs. Linux does not remap the nodemaskwhen the thread moves to a different cpuset context, nor when the set of nodes allowed by the thread's current cpuset context changes.
- MPOL_F_RELATIVE_NODES (since Linux-2.6.26)
- A nonempty nodemaskspecifies node IDs that are relative to the set of node IDs allowed by the thread's current cpuset.
nodemaskpoints to a bit mask of nodes containing up to maxnodebits. The bit mask size is rounded to themultiple of sizeof(unsigned long), but the kernel will use bits only up to maxnode. A NULL value of nodemaskor a maxnodevalue of zero specifies the empty set of nodes. If the value of maxnodeis zero, the nodemaskargument is ignored. Where a nodemaskis required, it must contain at least one node that is on-line, allowed by the thread's current cpuset context (unless the MPOL_F_STATIC_NODESmode flag is specified), and contains memory.
The modeargument must include one of the following values:
- This mode requests that any nondefault policy be removed, restoring default behavior. When applied to a range of memory via mbind(), this means to use the thread memory policy, which may have been set with set_mempolicy(2). If the mode of the thread memory policy is also MPOL_DEFAULT, the system-wide default policy will be used. The system-wide default policy allocates pages on the node of the CPU that triggers the allocation. For MPOL_DEFAULT, the nodemaskand maxnodearguments must be specify the empty set of nodes.
- This mode specifies a strict policy that restricts memory allocation to the nodes specified in nodemask. If nodemaskspecifies more than one node, page allocations will come from the node with sufficient free memory that is closest to the node where the allocation takes place. Pages will not be allocated from any node not specified in the IR nodemask . (Before Linux 2.6.26, page allocations came from the node with the lowest numeric node ID first, until that node contained no free memory. Allocations then came from the node with the highest node ID specified in nodemaskand so forth, until none of the specified nodes contained free memory.)
- This 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 1 MB 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.
- This mode 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 nodemaskspecifies more than one node ID, the first node in the mask will be selected as the preferred node. If the nodemaskand maxnodearguments specify the empty set, then the memory is allocated on the node of the CPU that triggered the allocation.
- 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 nodemaskand maxnodearguments 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 thread'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 thread's current cpuset context. By contrast, MPOL_DEFAULTreverts to the memory policy of the thread (which may be set via set_mempolicy(2)); that policy may be something other than "local allocation".
If MPOL_MF_STRICTis passed in flagsand modeis not MPOL_DEFAULT, then the call will fail with the error EIOif the existing pages in the memory range don't follow the policy.
If MPOL_MF_MOVEis 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_STRICTis also specified, then the call will fail with the error EIOif some pages could not be moved.
If MPOL_MF_MOVE_ALLis 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 thread must be privileged (CAP_SYS_NICE) to use this flag. If MPOL_MF_STRICTis also specified, then the call will fail with the error EIOif some pages could not be moved.
RETURN VALUEOn success, mbind() returns 0; on error, -1 is returned and errnois set to indicate the error.
- Part or all of the memory range specified by nodemaskand maxnodepoints outside your accessible address space. Or, there was an unmapped hole in the specified memory range specified by addrand len.
- An invalid value was specified for flagsor mode; or addr + lenwas less than addr; or addris not a multiple of the system page size. Or, modeis MPOL_DEFAULTand nodemaskspecified a nonempty set; or modeis MPOL_BINDor MPOL_INTERLEAVEand nodemaskis empty. Or, maxnodeexceeds a kernel-imposed limit. Or, nodemaskspecifies one or more node IDs that are greater than the maximum supported node ID. Or, none of the node IDs specified by nodemaskare on-line and allowed by the thread's current cpuset context, or none of the specified nodes contain memory. Or, the modeargument specified both MPOL_F_STATIC_NODESand MPOL_F_RELATIVE_NODES.
- MPOL_MF_STRICTwas specified and an existing page was already on a node that does not follow the policy; or MPOL_MF_MOVEor MPOL_MF_MOVE_ALLwas specified and the kernel was unable to move all existing pages in the range.
- Insufficient kernel memory was available.
- The flagsargument included the MPOL_MF_MOVE_ALLflag and the caller does not have the CAP_SYS_NICEprivilege.
VERSIONSThe mbind() system call was added to the Linux kernel in version 2.6.7.
CONFORMING TOThis system call is Linux-specific.
NOTESFor information on library support, see numa(7).
NUMA policy is not supported on a memory-mapped file range that was mapped with the MAP_SHAREDflag.
The MPOL_DEFAULTmode can have different effects for mbind() and set_mempolicy(2). When MPOL_DEFAULTis specified for set_mempolicy(2), the thread's memory policy reverts to the system default policy or local allocation. When MPOL_DEFAULTis specified for a range of memory using mbind(), any pages subsequently allocated for that range will use the thread's memory policy, as set by set_mempolicy(2). This effectively removes the explicit policy from the specified range, "falling back" to a possibly nondefault policy. To select explicit "local allocation" for a memory range, specify a modeof MPOL_LOCALor MPOL_PREFERREDwith an empty set of nodes. This method will work for set_mempolicy(2), as well.
Support for huge page policy was added with 2.6.16. For interleave policy to be effective on huge page mappings the policied memory needs to be tens of megabytes or larger.
MPOL_MF_STRICTis ignored on huge page mappings.
SEE ALSOget_mempolicy(2), getcpu(2), mmap(2), set_mempolicy(2), shmat(2), shmget(2), numa(3), cpuset(7), numa(7), numactl(8)
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