keras_distributed.py

import tensorflow as tf
import keras

# Flags for defining the tf.train.ClusterSpec
tf.app.flags.DEFINE_string("ps_hosts", "",
                           "Comma-separated list of hostname:port pairs")
tf.app.flags.DEFINE_string("worker_hosts", "",
                           "Comma-separated list of hostname:port pairs")

# Flags for defining the tf.train.Server
tf.app.flags.DEFINE_string("job_name", "", "One of 'ps', 'worker'")
tf.app.flags.DEFINE_integer("task_index", 0, "Index of task within the job")

FLAGS = tf.app.flags.FLAGS


def main(_):
  ps_hosts = FLAGS.ps_hosts.split(",")
  worker_hosts = FLAGS.worker_hosts.split(",")

  # Create a cluster from the parameter server and worker hosts.
  cluster = tf.train.ClusterSpec({"ps": ps_hosts, "worker": worker_hosts})
  
  # Create and start a server for the local task.
  server = tf.train.Server(cluster,
                           job_name=FLAGS.job_name,
                           task_index=FLAGS.task_index)

  if FLAGS.job_name == "ps":
    server.join()
  elif FLAGS.job_name == "worker":
    # Assigns ops to the local worker by default.
    with tf.device(tf.train.replica_device_setter(
        worker_device="/job:worker/task:%d" % FLAGS.task_index,
        cluster=cluster)):

      # set Keras learning phase to train
      keras.backend.set_learning_phase(1)
      # do not initialize variables on the fly
      keras.backend.manual_variable_initialization(True)

      # Build Keras model
      model = ...

      # keras model predictions
      preds = model.output
      # placeholder for training targets
      targets = tf.placeholder(...)
      # our categorical crossentropy loss
      xent_loss = tf.reduce_mean(
          keras.objectives.categorical_crossentropy(targets, preds))

      # we create a global_step tensor for distributed training
      # (a counter of iterations)
      global_step = tf.Variable(0, name='global_step', trainable=False)

      # apply regularizers if any
      if model.regularizers:
        total_loss = xent_loss * 1.  # copy tensor
        for regularizer in model.regularizers:
          total_loss = regularizer(total_loss)
      else:
        total_loss = xent_loss

      # set up TF optimizer
      optimizer = tf.train.RMSPropOptimizer(
            learning_rate, decay=0.9, momentum=FLAGS.momentum, epsilon=1e-8)

      # Set up model update ops (batch norm ops).
      # The gradients should only be computed after updating the moving average
      # of the batch normalization parameters, in order to prevent a data race
      # between the parameter updates and moving average computations.
      with tf.control_dependencies(model.updates):
        barrier = tf.no_op(name='update_barrier')

      # define gradient updates
      with tf.control_dependencies([barrier]):
        grads = optimizer.compute_gradients(
            total_loss,
            model.trainable_weights,
            gate_gradients=tf.Optimizer.GATE_OP,
            aggregation_method=None,
            colocate_gradients_with_ops=False)

      # define train tensor
      train_tensor = tf.with_dependencies([grad_updates],
                                          total_loss,
                                          name='train')

      # blah blah
      saver = tf.train.Saver()
      summary_op = tf.merge_all_summaries()
      init_op = tf.initialize_all_variables()

      # Create a "supervisor", which oversees the training process.
      sv = tf.train.Supervisor(is_chief=(FLAGS.task_index == 0),
                               logdir="/tmp/train_logs",
                               init_op=init_op,
                               summary_op=summary_op,
                               saver=saver,
                               global_step=global_step,
                               save_model_secs=600)

      # The supervisor takes care of session initialization, restoring from
      # a checkpoint, and closing when done or an error occurs.
      with sv.managed_session(server.target) as sess:
        # Loop until the supervisor shuts down or 1000000 steps have completed.
        step = 0
        while not sv.should_stop() and step < 1000000:
          # Run a training step asynchronously.
          # See `tf.train.SyncReplicasOptimizer` for additional details on how to
          # perform *synchronous* training.

          # feed_dict must contain the model inputs (the tensors listed in model.inputs)
          # and the "targets" placeholder we created ealier
          # it's a dictionary mapping tensors to batches of Numpy data
          # like: feed_dict={model.inputs[0]: np_train_data_batch, targets: np_train_labels_batch}
          loss_value, step_value = sess.run([train_op, global_step], feed_dict={...})

      # Ask for all the services to stop.
      sv.stop()

if __name__ == "__main__":
  tf.app.run()

@jerrypaytm,Embedding layer's parameters
model = Sequential()
model.add(Embedding(input_dim=1000, output_dim=64, input_length=10))
the model will take as input an integer matrix of size (batch, input_length).
the largest integer (i.e. word index) in the input should be no larger than 999 (vocabulary size).
now model.output_shape == (None, 10, 64), where None is the batch dimension

for this example, your input data can be something like:
data_example = numpy.array([[1,2,3,4,5,6,7,8,9,10], [1,2,3,4,5,6,7,8,9,10]]), the shape of data_example is (2,10)(in this example, the batch is 2) and the largest element of input data_example is 10 which is smaller than 1000, so it satisfies.

@THUfl12 Thanks for your help. I was fooled by the model checkpoint. Once I deleted the checkpoint, it refreshed some dimensions of the embedding to match the expected dictionary size.

Confused if this is the official way for data parallelism as well. According to this blog (https://blog.keras.io/keras-as-a-simplified-interface-to-tensorflow-tutorial.html) it's as simple as:

`server = tf.train.Server.create_local_server()
sess = tf.Session(server.target)

from keras import backend as K
K.set_session(sess)`

Can anyone confirm this method from the blog works?

@vagarwal87 the below code needs to be liked to the cluster. In short keras_distributed.py is the official way to do this.

`server = tf.train.Server.create_local_server()
sess = tf.Session(server.target)

from keras import backend as K
K.set_session(sess)`