get_mempolicy - retrieve NUMA memory policy for a thread

#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 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).

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

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