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

NAME

       mbind - set memory policy for a memory range

LIBRARY

       NUMA (Non-Uniform Memory Access) policy library (libnuma, -lnuma)

SYNOPSIS

       #include <numaif.h>

       long mbind(void addr[.len], unsigned long len, int mode,
                  const unsigned long nodemask[(.maxnode + ULONG_WIDTH - 1)
                                               / ULONG_WIDTH],
                  unsigned long maxnode, unsigned int flags);

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 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_SHARED mappings 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_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,
MPOL_WEIGHTED_INTERLEAVE, MPOL_PREFERRED, or MPOL_LOCAL (which are described in detail
below). All policy modes except MPOL_DEFAULT require the caller to specify the node or
nodes to which the mode applies, via the nodemask argument.

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

MPOL_F_NUMA_BALANCING (since Linux 5.15)
When mode is MPOL_BIND, enable the kernel NUMA balancing for the task if it is
supported by the kernel. If the flag isn't supported by the kernel, or is used
with mode other than MPOL_BIND, -1 is returned and errno is set to EINVAL.

MPOL_F_STATIC_NODES (since Linux-2.6.26)
A nonempty nodemask specifies physical node IDs. Linux does not remap the nodemask
when 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 nodemask specifies node IDs that are relative to the set of node IDs
allowed by the thread'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 thread's current cpuset context (unless the MPOL_F_STATIC_NODES mode flag is
specified), and contains memory.

The mode argument must include one of the following values:

MPOL_DEFAULT
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 nodemask and maxnode
arguments must be specify the empty set of nodes.

MPOL_BIND
This 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 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 next highest node ID
specified in nodemask and so forth, until none of the specified nodes contained
free memory.)

MPOL_INTERLEAVE
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.

MPOL_WEIGHTED_INTERLEAVE (since Linux 6.9)
This mode interleaves page allocations across the nodes specified in nodemask
according to the weights in /sys/kernel/mm/mempolicy/weighted_interleave. For
example, if bits 0, 2, and 5 are set in nodemask, and the contents of
/sys/kernel/mm/mempolicy/weighted_interleave/node0, /sys/.../node2, and
/sys/.../node5 are 4, 7, and 9, respectively, then pages in this region will be
allocated on nodes 0, 2, and 5 in a 4:7:9 ratio.

MPOL_PREFERRED
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 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.

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 nodemask and maxnode
arguments 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_DEFAULT reverts 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_STRICT is passed in flags and mode is not MPOL_DEFAULT, then the call fails
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 fails with the error EIO if some pages could not be moved. If the MPOL_INTERLEAVE
policy was specified, pages already residing on the specified nodes will not be moved such
that they are interleaved.

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 thread must be privileged (CAP_SYS_NICE) to use this flag. If MPOL_MF_STRICT is
also specified, then the call fails with the error EIO if some pages could not be moved.
If the MPOL_INTERLEAVE policy was specified, pages already residing on the specified nodes
will not be moved such that they are interleaved.

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
thread'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.

STANDARDS

       Linux.

HISTORY

       Linux 2.6.7.

       Support for huge page policy was added with Linux 2.6.16.  For  interleave  policy  to  be
       effective  on  huge  page  mappings  the  policied memory needs to be tens of megabytes or
       larger.

       Before Linux 5.7.  MPOL_MF_STRICT was ignored on huge page mappings.

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

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 thread's memory policy reverts to  the
       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
       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 mode of MPOL_LOCAL or
       MPOL_PREFERRED with an empty set of nodes.  This method will work for set_mempolicy(2), as
       well.