get_mempolicy - retrieve NUMA memory policy for a thread
NUMA (Non-Uniform Memory Access) policy library (
libnuma,
-lnuma)
#include <numaif.h>
long get_mempolicy(int *mode,
unsigned long nodemask[(.maxnode + ULONG_WIDTH - 1)
/ ULONG_WIDTH],
unsigned long maxnode, void *addr,
unsigned long flags);
get_mempolicy() retrieves the NUMA policy of the calling thread or of a
memory address, depending on the setting of
flags.
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.
If
flags is specified as 0, then information about the calling thread's
default policy (as set by
set_mempolicy(2)) is returned, in the buffers
pointed to by
mode and
nodemask. The value returned in these
arguments may be used to restore the thread's policy to its state at the time
of the call to
get_mempolicy() using
set_mempolicy(2). When
flags is 0,
addr must be specified as NULL.
If
flags specifies
MPOL_F_MEMS_ALLOWED (available since Linux
2.6.24), the
mode argument is ignored and the set of nodes (memories)
that the thread is allowed to specify in subsequent calls to
mbind(2)
or
set_mempolicy(2) (in the absence of any
mode flags) is
returned in
nodemask. It is not permitted to combine
MPOL_F_MEMS_ALLOWED with either
MPOL_F_ADDR or
MPOL_F_NODE.
If
flags specifies
MPOL_F_ADDR, then information is returned about
the policy governing the memory address given in
addr. This policy may
be different from the thread's default policy if
mbind(2) or one of the
helper functions described in
numa(3) has been used to establish a
policy for the memory range containing
addr.
If the
mode argument is not NULL, then
get_mempolicy() will store
the policy mode and any optional
mode flags of the requested NUMA
policy in the location pointed to by this argument. If
nodemask is not
NULL, then the nodemask associated with the policy will be stored in the
location pointed to by this argument.
maxnode specifies the number of
node IDs that can be stored into
nodemask—that is, the maximum
node ID plus one. The value specified by
maxnode is always rounded to a
multiple of
sizeof(unsigned long)*8.
If
flags specifies both
MPOL_F_NODE and
MPOL_F_ADDR,
get_mempolicy() will return the node ID of the node on which the
address
addr is allocated into the location pointed to by
mode.
If no page has yet been allocated for the specified address,
get_mempolicy() will allocate a page as if the thread had performed a
read (load) access to that address, and return the ID of the node where that
page was allocated.
If
flags specifies
MPOL_F_NODE, but not
MPOL_F_ADDR, and
the thread's current policy is
MPOL_INTERLEAVE, then
get_mempolicy() will return in the location pointed to by a non-NULL
mode argument, the node ID of the next node that will be used for
interleaving of internal kernel pages allocated on behalf of the thread. These
allocations include pages for memory-mapped files in process memory ranges
mapped using the
mmap(2) call with the
MAP_PRIVATE flag for read
accesses, and in memory ranges mapped with the
MAP_SHARED flag for all
accesses.
Other flag values are reserved.
For an overview of the possible policies see
set_mempolicy(2).
On success,
get_mempolicy() returns 0; on error, -1 is returned and
errno is set to indicate the error.
- EFAULT
- Part of all of the memory range specified by
nodemask and maxnode points outside your accessible address
space.
- EINVAL
- The value specified by maxnode is less than the
number of node IDs supported by the system. Or flags specified
values other than MPOL_F_NODE or MPOL_F_ADDR; or
flags specified MPOL_F_ADDR and addr is NULL, or
flags did not specify MPOL_F_ADDR and addr is not
NULL. Or, flags specified MPOL_F_NODE but not
MPOL_F_ADDR and the current thread policy is not
MPOL_INTERLEAVE. Or, flags specified
MPOL_F_MEMS_ALLOWED with either MPOL_F_ADDR or
MPOL_F_NODE. (And there are other EINVAL cases.)
The
get_mempolicy() system call was added in Linux 2.6.7.
This system call is Linux-specific.
For information on library support, see
numa(7).
getcpu(2),
mbind(2),
mmap(2),
set_mempolicy(2),
numa(3),
numa(7),
numactl(8)