<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def start(port, root_directory, bucket_depth):
 """Starts the mock S3 server on the given port at the given path."""

    application = S3Application(root_directory, bucket_depth)
    http_server = httpserver.HTTPServer(application)
    http_server.listen(port)
    ioloop.IOLoop.current().start()