def _find_orphaned_instances(self):
        """Given the set of instances and migrations already account for
        by resource tracker, sanity check the hypervisor to determine
        if there are any "orphaned" instances left hanging around.

        Orphans could be consuming memory and should be accounted for in
        usage calculations to guard against potential out of memory
        errors.
        """
        uuids1 = frozenset(self.tracked_instances.keys())
        uuids2 = frozenset(self.tracked_migrations.keys())
        uuids = uuids1 | uuids2

        usage = self.driver.get_per_instance_usage()
        vuuids = frozenset(usage.keys())

        orphan_uuids = vuuids - uuids
        orphans = [usage[uuid] for uuid in orphan_uuids]

        return orphans
dataset/ETHPy150Open openstack/nova/nova/compute/resource_tracker.py/ResourceTracker._find_orphaned_instances/original