Ubuntu Manpage: get_mempolicy - retrieve NUMA memory policy for a thread

NAME

       get_mempolicy - retrieve NUMA memory policy for a thread

LIBRARY

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

SYNOPSIS

       #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);

DESCRIPTION

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
or MPOL_WEIGHTED_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).

RETURN VALUE

       On success, get_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 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  neither  MPOL_INTERLEAVE  nor
              MPOL_WEIGHTED_INTERLEAVE.   Or,  flags  specified  MPOL_F_MEMS_ALLOWED  with either MPOL_F_ADDR or
              MPOL_F_NODE.  (And there are other EINVAL cases.)

STANDARDS

       Linux.

HISTORY

       Linux 2.6.7.

NOTES

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