Return the elapsed time since last update.

def getTimeSinceLastUpdate(IOType):
    """Return the elapsed time since last update."""
    global last_update_times
    # assert(IOType in ['net', 'disk', 'process_disk'])
    current_time = time()
    last_time = last_update_times.get(IOType)
    if not last_time:
        time_since_update = 1
    else:
        time_since_update = current_time - last_time
    last_update_times[IOType] = current_time
    return time_since_update