tf.train.replica_device_setter 'worker_device' parameter

According to the documentation https://www.tensorflow.org/api_docs/python/tf/train/replica_device_setter: By default, only Variable ops are placed on ps tasks, and the placement strategy is round-robin over all ps tasks.

Question: If 'worker_device' is supplied, then where do the 'Variable' ops live? On the worker or on ps.