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

NAME

       get_mempolicy - retrieve NUMA memory policy for a thread

SYNOPSIS

       #include <numaif.h>

       long get_mempolicy(int *mode, unsigned long *nodemask,
                         unsigned long maxnode, void *addr,
                         unsigned long flags);

       Link with -lnuma.

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, 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   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.)

VERSIONS

       The get_mempolicy() system call was added to the Linux kernel in version 2.6.7.

CONFORMING TO

       This system call is Linux-specific.

NOTES

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

COLOPHON

       This  page  is  part of release 4.15 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.