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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.

       Note: There is no glibc wrapper for this system call; see NOTES.

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() us‐
       ing 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
       Glibc does not provide a wrapper for this system call.  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                                                                          2021-03-22                                                               GET_MEMPOLICY(2)