title: Keras fit generator not use GPU in Tensorflow backend
body: <p>I have a code implemented with <code>Keras</code> and I am using the <code>fit_generator</code> because the size of the data is pretty big. </p>
  
  <p>Here is my code:</p>
  
  <pre><code>model = ad.kerasCNN() 
  
  model.fit_generator(
      ad.data_generator(X_TRAIN_metadata, Y_TRAIN_metadata, batch_size), 
      steps_per_epoch, 
      epochs=maxEpoch, 
      verbose=1, 
      callbacks=[history, early_stopping], 
      validation_data=ad.data_generator(X_DEV_metadata, Y_DEV_metadata, batch_size), 
      validation_steps=steps_per_epoch_dev, 
      class_weight=None, 
      max_queue_size=10, 
      workers=1, 
      use_multiprocessing=False, 
      initial_epoch=0
  )
  </code></pre>
  
  <p>When I run this code with a <strong>GPU</strong> I don't face any issue during the training phase, the GPU being used at between 70 - 80%. </p>
  
  <p>But when the code goes into the validation, the GPU is not used.
  This implies that the validation time is 23 times longer than the training phase. </p>
  
  <p>Is it a normal behavior of keras / tensorflow? </p>
  
  <p>What can I do to use the GPU in validation time?</p>