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GET_MEMPOLICY(2) Linux Programmer's Manual GET_MEMPOLICY(2)

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  dis-
     tances  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 pol-
     icy 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 multi-
     ple 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  allo-
     cated  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  maxn-
            ode 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_INTER-
            LEAVE.   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 ver-
     sion 2.6.7.

CONFORMING TO

     This system call is Linux-specific.

NOTES

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