set_mempolicy (2)
Leading comments
Copyright 2003,2004 Andi Kleen, SuSE Labs. and Copyright 2007 Lee Schermerhorn, Hewlett Packard %%%LICENSE_START(VERBATIM_PROF) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a ...
NAME
set_mempolicy - set default NUMA memory policy for a thread and its childrenSYNOPSIS
#include <numaif.h>
long set_mempolicy(int mode, const unsigned long *nodemask,
unsigned long maxnode);
Link with -lnuma.
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 distances 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 outside 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_INTERLEAVE, or MPOL_PREFERRED. All modes except MPOL_DEFAULT require the caller to specify via the nodemask argument one or more nodes.
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 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 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 node IDs that contains 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.
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
MPOL_DEFAULT
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 allocation"---that is,
allocate memory on the node of the CPU that triggered the allocation.
nodemask
must be specified as NULL.
If the "local node" contains no free memory, the system will
attempt to allocate memory from a "near by" node.
The
MPOL_BIND
mode defines 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.
MPOL_INTERLEAVE
interleaves page allocations across the nodes specified 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, accesses to a single page 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 "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).
The thread memory policy is preserved across an
execve(2),
and is inherited by child threads created using
fork(2)
or
clone(2).
RETURN VALUE
On success, set_mempolicy() returns 0; on error, -1 is returned and errno is set to indicate the error.ERRORS
- EFAULT
- Part of all of the memory range specified by nodemask and maxnode 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.
- ENOMEM
- Insufficient kernel memory was available.
VERSIONS
The set_mempolicy(), system call was added to the Linux kernel in version 2.6.7.CONFORMING TO
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).