get_mempolicy(2)                                        System Calls Manual                                        get_mempolicy(2)

       get_mempolicy - retrieve NUMA memory policy for a thread

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

       #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);

       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  re‐
       turned,  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

       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  re‐
       quested 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  ad‐
       dress  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_mempol‐
       icy() 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).

       On success, get_mempolicy() returns 0; on error, -1 is returned and errno is set to indicate the error.

       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 val‐
              ues 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.)

       The get_mempolicy() system call was added in Linux 2.6.7.

       This system call is Linux-specific.

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

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

Linux man-pages 6.03                                         2023-02-05                                            get_mempolicy(2)