set_mempolicy(2) System Calls Manual set_mempolicy(2)
NAME
set_mempolicy - set default NUMA memory policy for a thread and its
children
LIBRARY
NUMA (Non-Uniform Memory Access) policy library (libnuma, -lnuma)
SYNOPSIS
#include <numaif.h>
long set_mempolicy(int mode, const unsigned long *nodemask,
unsigned long maxnode);
DESCRIPTION
set_mempolicy() sets the NUMA memory policy of the calling thread,
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 dis-
tances to specific CPUs. The memory policy defines from which node
memory is allocated for the thread.
This system call defines the default policy for the thread. The thread
policy governs allocation of pages in the process's address space out-
side of memory ranges controlled by a more specific policy set by
mbind(2). The thread 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 thread and
of memory-mapped files mapped using the mmap(2) call with the
MAP_SHARED flag, regardless of the access type. The policy is applied
only when a new page is allocated for the thread. For anonymous memory
this is when the page is first touched by the thread.
The mode argument must specify one of MPOL_DEFAULT, MPOL_BIND, MPOL_IN-
TERLEAVE, MPOL_PREFERRED, or MPOL_LOCAL (which are described in detail
below). All 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 sup-
ported mode flags are:
MPOL_F_NUMA_BALANCING (since Linux 5.12)
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 sup-
ported by the kernel, or is used with mode other than MPOL_BIND,
-1 is returned and errno is set to EINVAL.
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.
MPOL_F_STATIC_NODES (since Linux 2.6.26)
A nonempty nodemask specifies physical node IDs. Linux will not
remap the nodemask when the process moves to a different cpuset
context, nor when the set of nodes allowed by the process's cur-
rent cpuset context changes.
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(un-
signed 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 ig-
nored.
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. If
the MPOL_F_STATIC_NODES is set in mode and a required nodemask contains
no nodes that are allowed by the process's current cpuset context, the
memory policy reverts to local allocation. This effectively overrides
the specified policy until the process's cpuset context includes one or
more of the nodes specified by nodemask.
The mode argument must include one of the following values:
MPOL_DEFAULT
This mode specifies that any nondefault thread memory policy be
removed, so that the memory policy "falls back" to the system
default policy. The system default policy is "local alloca-
tion"—that is, allocate memory on the node of the CPU that trig-
gered the allocation. nodemask must be specified as NULL. If
the "local node" contains no free memory, the system will at-
tempt to allocate memory from a "near by" node.
MPOL_BIND
This mode defines a strict policy that restricts memory alloca-
tion 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, un-
til none of the specified nodes contain free memory. Pages will
not be allocated from any node not specified in the nodemask.
MPOL_INTERLEAVE
This mode interleaves page allocations across the nodes speci-
fied 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, ac-
cesses to a single page will still be limited to the memory
bandwidth of a single node.
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
"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 policy
specifies "local allocation" (like the system default policy
discussed above).
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 ker-
nel 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 process's current cpuset context, the kernel will try to al-
locate memory from other nodes. The kernel will allocate memory
from the "local node" whenever it becomes allowed by the
process's current cpuset context.
The thread memory policy is preserved across an execve(2), and is in-
herited by child threads created using fork(2) or clone(2).
RETURN VALUE
On success, set_mempolicy() returns 0; on error, -1 is returned and er-
rno is set to indicate the error.
ERRORS
EFAULT Part of all of the memory range specified by nodemask and maxn-
ode points outside your accessible address space.
EINVAL mode is invalid. Or, mode is MPOL_DEFAULT and nodemask is
nonempty, or mode is MPOL_BIND or MPOL_INTERLEAVE and nodemask
is empty. Or, maxnode specifies more than a page worth of bits.
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. Or, the
MPOL_F_NUMA_BALANCING isn't supported by the kernel, or is used
with mode other than MPOL_BIND.
ENOMEM Insufficient kernel memory was available.
VERSIONS
The set_mempolicy() system call was added in Linux 2.6.7.
STANDARDS
This system call is Linux-specific.
NOTES
Memory 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 thread or memory
range that is in effect at the time the page is allocated.
For information on library support, see numa(7).
SEE ALSO
get_mempolicy(2), getcpu(2), mbind(2), mmap(2), numa(3), cpuset(7),
numa(7), numactl(8)
Linux man-pages 6.03 2023-02-05 set_mempolicy(2)
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