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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,
       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.)

STANDARDS

       Linux.

HISTORY

       Linux 2.6.7.

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)

Linux man-pages 6.7                                2023-10-31                                   get_mempolicy(2)