如何在TensorFlow中使用批处理规范化？

我想在TensorFlow中使用批处理规范化。我在中找到了相关的C ++源代码core/ops/nn_ops.cc。但是，我没有在tensorflow.org上找到它的文档。

BN在MLP和CNN中具有不同的语义，因此我不确定此BN的确切作用。

我没有找到任何一种方法MovingMoments。

2016年7月更新 在TensorFlow中使用批处理规范化的最简单方法是通过contrib / layers，tflearn或slim中提供的高级接口。

如果您想自己动手，则可以使用先前的答案：自发布以来，此文档的文档字符串已得到改进-请参阅master分支中的docs注释，而不是找到的注释。它特别说明了它是的输出tf.nn.moments。

您可以在batch_norm测试代码中看到一个非常简单的示例。对于更真实的使用示例，我将其包含在帮助器类下面，并使用了我自己编写的注释（不提供保修！）：

"""A helper class for managing batch normalization state.                   

This class is designed to simplify adding batch normalization               
(http://arxiv.org/pdf/1502.03167v3.pdf) to your model by                    
managing the state variables associated with it.                            

Important use note:  The function get_assigner() returns                    
an op that must be executed to save the updated state.                      
A suggested way to do this is to make execution of the                      
model optimizer force it, e.g., by:                                         

  update_assignments = tf.group(bn1.get_assigner(),                         
                                bn2.get_assigner())                         
  with tf.control_dependencies([optimizer]):                                
    optimizer = tf.group(update_assignments)                                

"""

import tensorflow as tf


class ConvolutionalBatchNormalizer(object):
  """Helper class that groups the normalization logic and variables.        

  Use:                                                                      
      ewma = tf.train.ExponentialMovingAverage(decay=0.99)                  
      bn = ConvolutionalBatchNormalizer(depth, 0.001, ewma, True)           
      update_assignments = bn.get_assigner()                                
      x = bn.normalize(y, train=training?)                                  
      (the output x will be batch-normalized).                              
  """

  def __init__(self, depth, epsilon, ewma_trainer, scale_after_norm):
    self.mean = tf.Variable(tf.constant(0.0, shape=[depth]),
                            trainable=False)
    self.variance = tf.Variable(tf.constant(1.0, shape=[depth]),
                                trainable=False)
    self.beta = tf.Variable(tf.constant(0.0, shape=[depth]))
    self.gamma = tf.Variable(tf.constant(1.0, shape=[depth]))
    self.ewma_trainer = ewma_trainer
    self.epsilon = epsilon
    self.scale_after_norm = scale_after_norm

  def get_assigner(self):
    """Returns an EWMA apply op that must be invoked after optimization."""
    return self.ewma_trainer.apply([self.mean, self.variance])

  def normalize(self, x, train=True):
    """Returns a batch-normalized version of x."""
    if train:
      mean, variance = tf.nn.moments(x, [0, 1, 2])
      assign_mean = self.mean.assign(mean)
      assign_variance = self.variance.assign(variance)
      with tf.control_dependencies([assign_mean, assign_variance]):
        return tf.nn.batch_norm_with_global_normalization(
            x, mean, variance, self.beta, self.gamma,
            self.epsilon, self.scale_after_norm)
    else:
      mean = self.ewma_trainer.average(self.mean)
      variance = self.ewma_trainer.average(self.variance)
      local_beta = tf.identity(self.beta)
      local_gamma = tf.identity(self.gamma)
      return tf.nn.batch_norm_with_global_normalization(
          x, mean, variance, local_beta, local_gamma,
          self.epsilon, self.scale_after_norm)

请注意，我ConvolutionalBatchNormalizer之所以称其为a是因为它tf.nn.moments在轴0、1和2上固定了使用sum的用途，而对于非卷积用途，您可能只需要轴0。

如果您使用它，反馈表示赞赏。

从TensorFlow 1.0（2017年2月）开始tf.layers.batch_normalization，TensorFlow本身还包含高级API。

使用起来超级简单：

# Set this to True for training and False for testing
training = tf.placeholder(tf.bool)

x = tf.layers.dense(input_x, units=100)
x = tf.layers.batch_normalization(x, training=training)
x = tf.nn.relu(x)

...除了它将额外的操作添加到图形中（用于更新其均值和方差变量）以使它们不会成为训练操作的依赖项。您可以单独运行操作：

extra_update_ops = tf.get_collection(tf.GraphKeys.UPDATE_OPS)
sess.run([train_op, extra_update_ops], ...)

或手动将更新操作作为您的培训操作的依赖项添加，然后像平常一样运行您的培训操作：

extra_update_ops = tf.get_collection(tf.GraphKeys.UPDATE_OPS)
with tf.control_dependencies(extra_update_ops):
    train_op = optimizer.minimize(loss)
...
sess.run([train_op], ...)

以下内容对我来说很好用，不需要在外部调用EMA-apply。

import numpy as np
import tensorflow as tf
from tensorflow.python import control_flow_ops

def batch_norm(x, n_out, phase_train, scope='bn'):
    """
    Batch normalization on convolutional maps.
    Args:
        x:           Tensor, 4D BHWD input maps
        n_out:       integer, depth of input maps
        phase_train: boolean tf.Varialbe, true indicates training phase
        scope:       string, variable scope
    Return:
        normed:      batch-normalized maps
    """
    with tf.variable_scope(scope):
        beta = tf.Variable(tf.constant(0.0, shape=[n_out]),
                                     name='beta', trainable=True)
        gamma = tf.Variable(tf.constant(1.0, shape=[n_out]),
                                      name='gamma', trainable=True)
        batch_mean, batch_var = tf.nn.moments(x, [0,1,2], name='moments')
        ema = tf.train.ExponentialMovingAverage(decay=0.5)

        def mean_var_with_update():
            ema_apply_op = ema.apply([batch_mean, batch_var])
            with tf.control_dependencies([ema_apply_op]):
                return tf.identity(batch_mean), tf.identity(batch_var)

        mean, var = tf.cond(phase_train,
                            mean_var_with_update,
                            lambda: (ema.average(batch_mean), ema.average(batch_var)))
        normed = tf.nn.batch_normalization(x, mean, var, beta, gamma, 1e-3)
    return normed

例：

import math

n_in, n_out = 3, 16
ksize = 3
stride = 1
phase_train = tf.placeholder(tf.bool, name='phase_train')
input_image = tf.placeholder(tf.float32, name='input_image')
kernel = tf.Variable(tf.truncated_normal([ksize, ksize, n_in, n_out],
                                   stddev=math.sqrt(2.0/(ksize*ksize*n_out))),
                                   name='kernel')
conv = tf.nn.conv2d(input_image, kernel, [1,stride,stride,1], padding='SAME')
conv_bn = batch_norm(conv, n_out, phase_train)
relu = tf.nn.relu(conv_bn)

with tf.Session() as session:
    session.run(tf.initialize_all_variables())
    for i in range(20):
        test_image = np.random.rand(4,32,32,3)
        sess_outputs = session.run([relu],
          {input_image.name: test_image, phase_train.name: True})

还有一个由开发人员编码的“官方”批处理规范化层。他们没有关于如何使用它的很好的文档，但是这里是如何使用它（根据我的说法）：

from tensorflow.contrib.layers.python.layers import batch_norm as batch_norm

def batch_norm_layer(x,train_phase,scope_bn):
    bn_train = batch_norm(x, decay=0.999, center=True, scale=True,
    updates_collections=None,
    is_training=True,
    reuse=None, # is this right?
    trainable=True,
    scope=scope_bn)
    bn_inference = batch_norm(x, decay=0.999, center=True, scale=True,
    updates_collections=None,
    is_training=False,
    reuse=True, # is this right?
    trainable=True,
    scope=scope_bn)
    z = tf.cond(train_phase, lambda: bn_train, lambda: bn_inference)
    return z

要实际使用它，您需要为其创建一个占位符，train_phase以指示您是否处于训练或推断阶段（如中所示train_phase = tf.placeholder(tf.bool, name='phase_train')）。它的值可以在推理或训练过程中用来填充，tf.session例如：

test_error = sess.run(fetches=cross_entropy, feed_dict={x: batch_xtest, y_:batch_ytest, train_phase: False})

或在训练期间：

sess.run(fetches=train_step, feed_dict={x: batch_xs, y_:batch_ys, train_phase: True})

根据github中的讨论，我很确定这是正确的。

似乎还有另一个有用的链接：

http://r2rt.com/implementing-batch-normalization-in-tensorflow.html

您可以简单地使用内置的batch_norm层：

batch_norm = tf.cond(is_train, 
    lambda: tf.contrib.layers.batch_norm(prev, activation_fn=tf.nn.relu, is_training=True, reuse=None),
    lambda: tf.contrib.layers.batch_norm(prev, activation_fn =tf.nn.relu, is_training=False, reuse=True))

其中prev是上一层的输出（可以是全连接层或卷积层），而is_train是布尔占位符。只需使用batch_norm作为下一层的输入即可。

由于最近有人对此进行了编辑，因此我想澄清一下，这不再是一个问题。

答案似乎不正确当phase_train设置为false时，它仍然更新ema均值和方差。可以使用以下代码段对此进行验证。

x = tf.placeholder(tf.float32, [None, 20, 20, 10], name='input')
phase_train = tf.placeholder(tf.bool, name='phase_train')

# generate random noise to pass into batch norm
x_gen = tf.random_normal([50,20,20,10])
pt_false = tf.Variable(tf.constant(True))

#generate a constant variable to pass into batch norm
y = x_gen.eval()

[bn, bn_vars] = batch_norm(x, 10, phase_train)

tf.initialize_all_variables().run()
train_step = lambda: bn.eval({x:x_gen.eval(), phase_train:True})
test_step = lambda: bn.eval({x:y, phase_train:False})
test_step_c = lambda: bn.eval({x:y, phase_train:True})

# Verify that this is different as expected, two different x's have different norms
print(train_step()[0][0][0])
print(train_step()[0][0][0])

# Verify that this is same as expected, same x's (y) have same norm
print(train_step_c()[0][0][0])
print(train_step_c()[0][0][0])

# THIS IS DIFFERENT but should be they same, should only be reading from the ema.
print(test_step()[0][0][0])
print(test_step()[0][0][0])

下面是使用TensorFlow内置的batch_norm层的代码，用于加载数据，构建具有一个隐藏的ReLU层和L2标准化的网络，并为隐藏层和外部层引入批处理标准化。这样运行良好，训练也很好。仅供参考，此示例主要基于Udacity DeepLearning课程的数据和代码构建。PS是的，在较早的答案中已经对其中的一部分进行了讨论，但是我决定将所有代码都收集在一个代码段中，以便为您提供带有批处理规范化及其评估的整个网络培训过程的示例

# These are all the modules we'll be using later. Make sure you can import them
# before proceeding further.
from __future__ import print_function
import numpy as np
import tensorflow as tf
from six.moves import cPickle as pickle

pickle_file = '/home/maxkhk/Documents/Udacity/DeepLearningCourse/SourceCode/tensorflow/examples/udacity/notMNIST.pickle'

with open(pickle_file, 'rb') as f:
  save = pickle.load(f)
  train_dataset = save['train_dataset']
  train_labels = save['train_labels']
  valid_dataset = save['valid_dataset']
  valid_labels = save['valid_labels']
  test_dataset = save['test_dataset']
  test_labels = save['test_labels']
  del save  # hint to help gc free up memory
  print('Training set', train_dataset.shape, train_labels.shape)
  print('Validation set', valid_dataset.shape, valid_labels.shape)
  print('Test set', test_dataset.shape, test_labels.shape)

image_size = 28
num_labels = 10

def reformat(dataset, labels):
  dataset = dataset.reshape((-1, image_size * image_size)).astype(np.float32)
  # Map 2 to [0.0, 1.0, 0.0 ...], 3 to [0.0, 0.0, 1.0 ...]
  labels = (np.arange(num_labels) == labels[:,None]).astype(np.float32)
  return dataset, labels
train_dataset, train_labels = reformat(train_dataset, train_labels)
valid_dataset, valid_labels = reformat(valid_dataset, valid_labels)
test_dataset, test_labels = reformat(test_dataset, test_labels)
print('Training set', train_dataset.shape, train_labels.shape)
print('Validation set', valid_dataset.shape, valid_labels.shape)
print('Test set', test_dataset.shape, test_labels.shape)


def accuracy(predictions, labels):
  return (100.0 * np.sum(np.argmax(predictions, 1) == np.argmax(labels, 1))
          / predictions.shape[0])


#for NeuralNetwork model code is below
#We will use SGD for training to save our time. Code is from Assignment 2
#beta is the new parameter - controls level of regularization.
#Feel free to play with it - the best one I found is 0.001
#notice, we introduce L2 for both biases and weights of all layers

batch_size = 128
beta = 0.001

#building tensorflow graph
graph = tf.Graph()
with graph.as_default():
      # Input data. For the training data, we use a placeholder that will be fed
  # at run time with a training minibatch.
  tf_train_dataset = tf.placeholder(tf.float32,
                                    shape=(batch_size, image_size * image_size))
  tf_train_labels = tf.placeholder(tf.float32, shape=(batch_size, num_labels))
  tf_valid_dataset = tf.constant(valid_dataset)
  tf_test_dataset = tf.constant(test_dataset)

  #introduce batchnorm
  tf_train_dataset_bn = tf.contrib.layers.batch_norm(tf_train_dataset)


  #now let's build our new hidden layer
  #that's how many hidden neurons we want
  num_hidden_neurons = 1024
  #its weights
  hidden_weights = tf.Variable(
    tf.truncated_normal([image_size * image_size, num_hidden_neurons]))
  hidden_biases = tf.Variable(tf.zeros([num_hidden_neurons]))

  #now the layer itself. It multiplies data by weights, adds biases
  #and takes ReLU over result
  hidden_layer = tf.nn.relu(tf.matmul(tf_train_dataset_bn, hidden_weights) + hidden_biases)

  #adding the batch normalization layerhi()
  hidden_layer_bn = tf.contrib.layers.batch_norm(hidden_layer)

  #time to go for output linear layer
  #out weights connect hidden neurons to output labels
  #biases are added to output labels  
  out_weights = tf.Variable(
    tf.truncated_normal([num_hidden_neurons, num_labels]))  

  out_biases = tf.Variable(tf.zeros([num_labels]))  

  #compute output  
  out_layer = tf.matmul(hidden_layer_bn,out_weights) + out_biases
  #our real output is a softmax of prior result
  #and we also compute its cross-entropy to get our loss
  #Notice - we introduce our L2 here
  loss = (tf.reduce_mean(tf.nn.softmax_cross_entropy_with_logits(
    out_layer, tf_train_labels) +
    beta*tf.nn.l2_loss(hidden_weights) +
    beta*tf.nn.l2_loss(hidden_biases) +
    beta*tf.nn.l2_loss(out_weights) +
    beta*tf.nn.l2_loss(out_biases)))

  #now we just minimize this loss to actually train the network
  optimizer = tf.train.GradientDescentOptimizer(0.5).minimize(loss)

  #nice, now let's calculate the predictions on each dataset for evaluating the
  #performance so far
  # Predictions for the training, validation, and test data.
  train_prediction = tf.nn.softmax(out_layer)
  valid_relu = tf.nn.relu(  tf.matmul(tf_valid_dataset, hidden_weights) + hidden_biases)
  valid_prediction = tf.nn.softmax( tf.matmul(valid_relu, out_weights) + out_biases) 

  test_relu = tf.nn.relu( tf.matmul( tf_test_dataset, hidden_weights) + hidden_biases)
  test_prediction = tf.nn.softmax(tf.matmul(test_relu, out_weights) + out_biases)



#now is the actual training on the ANN we built
#we will run it for some number of steps and evaluate the progress after 
#every 500 steps

#number of steps we will train our ANN
num_steps = 3001

#actual training
with tf.Session(graph=graph) as session:
  tf.initialize_all_variables().run()
  print("Initialized")
  for step in range(num_steps):
    # Pick an offset within the training data, which has been randomized.
    # Note: we could use better randomization across epochs.
    offset = (step * batch_size) % (train_labels.shape[0] - batch_size)
    # Generate a minibatch.
    batch_data = train_dataset[offset:(offset + batch_size), :]
    batch_labels = train_labels[offset:(offset + batch_size), :]
    # Prepare a dictionary telling the session where to feed the minibatch.
    # The key of the dictionary is the placeholder node of the graph to be fed,
    # and the value is the numpy array to feed to it.
    feed_dict = {tf_train_dataset : batch_data, tf_train_labels : batch_labels}
    _, l, predictions = session.run(
      [optimizer, loss, train_prediction], feed_dict=feed_dict)
    if (step % 500 == 0):
      print("Minibatch loss at step %d: %f" % (step, l))
      print("Minibatch accuracy: %.1f%%" % accuracy(predictions, batch_labels))
      print("Validation accuracy: %.1f%%" % accuracy(
        valid_prediction.eval(), valid_labels))
      print("Test accuracy: %.1f%%" % accuracy(test_prediction.eval(), test_labels))

下面是使用这个batchnorm类的简单示例：

from bn_class import *

with tf.name_scope('Batch_norm_conv1') as scope:
    ewma = tf.train.ExponentialMovingAverage(decay=0.99)                  
    bn_conv1 = ConvolutionalBatchNormalizer(num_filt_1, 0.001, ewma, True)           
    update_assignments = bn_conv1.get_assigner() 
    a_conv1 = bn_conv1.normalize(a_conv1, train=bn_train) 
    h_conv1 = tf.nn.relu(a_conv1)