Ubuntu Manpage: mbind - set memory policy for a memory range

NAME

       mbind - set memory policy for a memory range

SYNOPSIS

       #include <numaif.h>

       long mbind(void *addr, unsigned long len, int mode,
                  const unsigned long *nodemask, unsigned long maxnode,
                  unsigned flags);

       Link with -lnuma.

DESCRIPTION

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

If the memory range specified by the addr 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 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 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
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_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 argument, the node or nodes to which the mode applies.

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 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 nodes containing 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.

The MPOL_DEFAULT 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 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 allocates 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 mode 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

EFAULT Part or all of the memory range specified by nodemask and maxnode points outside
your accessible address space. Or, there was an unmapped hole in the specified
memory range specified by addr and len.

EINVAL An invalid value was specified for flags or mode; or addr + len was less than addr;
or addr is not a multiple of the system page size. Or, mode is MPOL_DEFAULT and
nodemask specified a nonempty set; or mode is MPOL_BIND or MPOL_INTERLEAVE and
nodemask is empty. Or, maxnode exceeds a kernel-imposed limit. 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.

EIO MPOL_MF_STRICT was specified and an existing page was already on a node that does
not follow the policy; or MPOL_MF_MOVE or MPOL_MF_MOVE_ALL was specified and the
kernel was unable to move all existing pages in the range.

ENOMEM Insufficient kernel memory was available.

EPERM The flags argument included the MPOL_MF_MOVE_ALL flag and the caller does not have
the CAP_SYS_NICE privilege.

VERSIONS

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

CONFORMING TO

       This system call is Linux-specific.

NOTES

       For information on library support, see numa(7).

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

       The  MPOL_DEFAULT  mode can have different effects for mbind() and set_mempolicy(2).  When
       MPOL_DEFAULT is specified for set_mempolicy(2), the process's  policy  reverts  to  system
       default  policy or local allocation.  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, "falling back" to a  possibly  nondefault  policy.   To  select  explicit
       "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.

       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_STRICT is ignored on huge page mappings.

       MPOL_MF_MOVE and MPOL_MF_MOVE_ALL are available only on Linux 2.6.16 and later.

COLOPHON

       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.