get_mempolicy(2) — Linux manual page


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

NAME         top

       get_mempolicy - retrieve NUMA memory policy for a thread

SYNOPSIS         top

       #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         top

       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

       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

       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         top

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

ERRORS         top

       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         top

       The get_mempolicy() system call was added to the Linux kernel in
       version 2.6.7.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       Glibc does not provide a wrapper for this system call.  For
       information on library support, see numa(7).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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

Linux                          2021-03-22               GET_MEMPOLICY(2)

Pages that refer to this page: mbind(2)migrate_pages(2)move_pages(2)set_mempolicy(2)syscalls(2)numa(3)cpuset(7)numa(7)migratepages(8)numactl(8)