tensorflow多GPU（同步模式）

#%%
# Copyright 2016 The TensorFlow Authors. All Rights Reserved.
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
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#     http://www.apache.org/licenses/LICENSE-2.0
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# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
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# See the License for the specific language governing permissions and
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# ==============================================================================
import os.path
import re
import time
import numpy as np
import tensorflow as tf
import cifar10



batch_size=128
#train_dir='/tmp/cifar10_train'
max_steps=1000000
num_gpus=4
#log_device_placement=False


def tower_loss(scope):
  """Calculate the total loss on a single tower running the CIFAR model.

  Args:
    scope: unique prefix string identifying the CIFAR tower, e.g. 'tower_0'

  Returns:
     Tensor of shape [] containing the total loss for a batch of data
  """
  # Get images and labels for CIFAR-10.
  images, labels = cifar10.distorted_inputs()

  # Build inference Graph.
  logits = cifar10.inference(images)

  # Build the portion of the Graph calculating the losses. Note that we will
  # assemble the total_loss using a custom function below.
  _ = cifar10.loss(logits, labels)

  # Assemble all of the losses for the current tower only.
  losses = tf.get_collection('losses', scope)

  # Calculate the total loss for the current tower.
  total_loss = tf.add_n(losses, name='total_loss')

  # Compute the moving average of all individual losses and the total loss.
#  loss_averages = tf.train.ExponentialMovingAverage(0.9, name='avg')
#  loss_averages_op = loss_averages.apply(losses + [total_loss])

  # Attach a scalar summary to all individual losses and the total loss; do the
  # same for the averaged version of the losses.
#  for l in losses + [total_loss]:
    # Remove 'tower_[0-9]/' from the name in case this is a multi-GPU training
    # session. This helps the clarity of presentation on tensorboard.
#    loss_name = re.sub('%s_[0-9]*/' % cifar10.TOWER_NAME, '', l.op.name)
    # Name each loss as '(raw)' and name the moving average version of the loss
    # as the original loss name.
#    tf.scalar_summary(loss_name +' (raw)', l)
#    tf.scalar_summary(loss_name, loss_averages.average(l))

#  with tf.control_dependencies([loss_averages_op]):
#    total_loss = tf.identity(total_loss)
  return total_loss


def average_gradients(tower_grads):
  """Calculate the average gradient for each shared variable across all towers.

  Note that this function provides a synchronization point across all towers.

  Args:
    tower_grads: List of lists of (gradient, variable) tuples. The outer list
      is over individual gradients. The inner list is over the gradient
      calculation for each tower.
  Returns:
     List of pairs of (gradient, variable) where the gradient has been averaged
     across all towers.
  """
  average_grads = []
  for grad_and_vars in zip(*tower_grads):
    # Note that each grad_and_vars looks like the following:
    #   ((grad0_gpu0, var0_gpu0), ... , (grad0_gpuN, var0_gpuN))
    grads = []
    for g, _ in grad_and_vars:
      # Add 0 dimension to the gradients to represent the tower.
      expanded_g = tf.expand_dims(g, 0)

      # Append on a 'tower' dimension which we will average over below.
      grads.append(expanded_g)

    # Average over the 'tower' dimension.
    grad = tf.concat(grads, 0)
    grad = tf.reduce_mean(grad, 0)

    # Keep in mind that the Variables are redundant because they are shared
    # across towers. So .. we will just return the first tower's pointer to
    # the Variable.
    v = grad_and_vars[0][1]
    grad_and_var = (grad, v)
    average_grads.append(grad_and_var)
  return average_grads


def train():
  """Train CIFAR-10 for a number of steps."""
  with tf.Graph().as_default(), tf.device('/cpu:0'):
    # Create a variable to count the number of train() calls. This equals the
    # number of batches processed * FLAGS.num_gpus.
    global_step = tf.get_variable(
        'global_step', [],
        initializer=tf.constant_initializer(0), trainable=False)

    # Calculate the learning rate schedule.
    num_batches_per_epoch = (cifar10.NUM_EXAMPLES_PER_EPOCH_FOR_TRAIN /
                             batch_size)
    decay_steps = int(num_batches_per_epoch * cifar10.NUM_EPOCHS_PER_DECAY)

    # Decay the learning rate exponentially based on the number of steps.
    lr = tf.train.exponential_decay(cifar10.INITIAL_LEARNING_RATE,
                                    global_step,
                                    decay_steps,
                                    cifar10.LEARNING_RATE_DECAY_FACTOR,
                                    staircase=True)

    # Create an optimizer that performs gradient descent.
    opt = tf.train.GradientDescentOptimizer(lr)

    # Calculate the gradients for each model tower.
    tower_grads = []
    for i in range(num_gpus):
      with tf.device('/gpu:%d' % i):
        with tf.name_scope('%s_%d' % (cifar10.TOWER_NAME, i)) as scope:
          # Calculate the loss for one tower of the CIFAR model. This function
          # constructs the entire CIFAR model but shares the variables across
          # all towers.
          loss = tower_loss(scope)

          # Reuse variables for the next tower.
          tf.get_variable_scope().reuse_variables()

          # Retain the summaries from the final tower.
#          summaries = tf.get_collection(tf.GraphKeys.SUMMARIES, scope)

          # Calculate the gradients for the batch of data on this CIFAR tower.
          grads = opt.compute_gradients(loss)

          # Keep track of the gradients across all towers.
          tower_grads.append(grads)

    # We must calculate the mean of each gradient. Note that this is the
    # synchronization point across all towers.
    grads = average_gradients(tower_grads)

    # Add a summary to track the learning rate.
#    summaries.append(tf.scalar_summary('learning_rate', lr))

    # Add histograms for gradients.
#    for grad, var in grads:
#      if grad is not None:
#        summaries.append(
#            tf.histogram_summary(var.op.name + '/gradients', grad))

    # Apply the gradients to adjust the shared variables.
    apply_gradient_op = opt.apply_gradients(grads, global_step=global_step)

    # Add histograms for trainable variables.
#    for var in tf.trainable_variables():
#      summaries.append(tf.histogram_summary(var.op.name, var))

    # Track the moving averages of all trainable variables.
#    variable_averages = tf.train.ExponentialMovingAverage(
#        cifar10.MOVING_AVERAGE_DECAY, global_step)
#    variables_averages_op = variable_averages.apply(tf.trainable_variables())

    # Group all updates to into a single train op.
#    train_op = tf.group(apply_gradient_op, variables_averages_op)

    # Create a saver.
    saver = tf.train.Saver(tf.all_variables())

    # Build the summary operation from the last tower summaries.
#    summary_op = tf.merge_summary(summaries)

    # Build an initialization operation to run below.
    init = tf.global_variables_initializer()

    # Start running operations on the Graph. allow_soft_placement must be set to
    # True to build towers on GPU, as some of the ops do not have GPU
    # implementations.
    sess = tf.Session(config=tf.ConfigProto(allow_soft_placement=True))
    sess.run(init)

    # Start the queue runners.
    tf.train.start_queue_runners(sess=sess)

#    summary_writer = tf.train.SummaryWriter(train_dir, sess.graph)

    for step in range(max_steps):
      start_time = time.time()
      _, loss_value = sess.run([apply_gradient_op, loss])
      duration = time.time() - start_time

      assert not np.isnan(loss_value), 'Model diverged with loss = NaN'

      if step % 10 == 0:
        num_examples_per_step = batch_size * num_gpus
        examples_per_sec = num_examples_per_step / duration
        sec_per_batch = duration / num_gpus

        format_str = ('step %d, loss = %.2f (%.1f examples/sec; %.3f '
                      'sec/batch)')
        print (format_str % (step, loss_value,
                             examples_per_sec, sec_per_batch))

#      if step % 100 == 0:
#        summary_str = sess.run(summary_op)
#        summary_writer.add_summary(summary_str, step)

      # Save the model checkpoint periodically.
      if step % 1000 == 0 or (step + 1) == max_steps:
#        checkpoint_path = os.path.join(train_dir, 'model.ckpt')
        saver.save(sess, '/tmp/cifar10_train/model.ckpt', global_step=step)



cifar10.maybe_download_and_extract()
#if tf.gfile.Exists(train_dir):
#  tf.gfile.DeleteRecursively(train_dir)
#tf.gfile.MakeDirs(train_dir)
train()