title: Parallelism isn't reducing the time in dataset map
body: <p><a href="https://www.tensorflow.org/versions/r1.4/api_docs/python/tf/data/Dataset#map" rel="noreferrer">TF Map function supports parallel calls</a>. I'm seeing no improvements passing <code>num_parallel_calls</code> to map. With <code>num_parallel_calls=1</code> and <code>num_parallel_calls=10</code>, there is no improvement in performance run time. Here is a simple code</p>
  
  <pre><code>import time
  def test_two_custom_function_parallelism(num_parallel_calls=1, batch=False, 
      batch_size=1, repeat=1, num_iterations=10):
      tf.reset_default_graph()
      start = time.time()
      dataset_x = tf.data.Dataset.range(1000).map(lambda x: tf.py_func(
          squarer, [x], [tf.int64]), 
          num_parallel_calls=num_parallel_calls).repeat(repeat)
      if batch:
          dataset_x = dataset_x.batch(batch_size)
      dataset_y = tf.data.Dataset.range(1000).map(lambda x: tf.py_func(
         squarer, [x], [tf.int64]), num_parallel_calls=num_parallel_calls).repeat(repeat)
      if batch:
          dataset_y = dataset_x.batch(batch_size)
          X = dataset_x.make_one_shot_iterator().get_next()
          Y = dataset_x.make_one_shot_iterator().get_next()
  
      with tf.Session() as sess:
          sess.run(tf.global_variables_initializer())
          i = 0
          while True:
              try:
                  res = sess.run([X, Y])
                  i += 1
                  if i == num_iterations:
                      break
              except tf.errors.OutOfRangeError as e:
                  pass
  </code></pre>
  
  <p>Here are the timings</p>
  
  <pre><code>%timeit test_two_custom_function_parallelism(num_iterations=1000, 
   num_parallel_calls=2, batch_size=2, batch=True)
  370ms
  
  %timeit test_two_custom_function_parallelism(num_iterations=1000, 
   num_parallel_calls=5, batch_size=2, batch=True)
  372ms
  
  %timeit test_two_custom_function_parallelism(num_iterations=1000, 
   num_parallel_calls=10, batch_size=2, batch=True)
  384ms
  </code></pre>
  
  <p>I used <code>%timeit</code> in Juypter notebook. What am I doing it wrong?</p>