NAME

       get_mempolicy - retrieve NUMA memory policy for a thread

SYNOPSIS

       #include <numaif.h>

       int get_mempolicy(int *mode, unsigned long *nodemask,
                         unsigned long maxnode, unsigned long 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.  The policy returned [mode and nodemask] 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).

       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).

       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).

SEE ALSO

       getcpu(2), mbind(2), mmap(2), set_mempolicy(2), numa(3), numa(7), numactl(8)

COLOPHON

       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.