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TensorFlow搭建神經網絡最佳實踐

2020-01-04 15:44:18
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一、TensorFLow完整樣例

在MNIST數據集上,搭建一個簡單神經網絡結構,一個包含ReLU單元的非線性化處理的兩層神經網絡。在訓練神經網絡的時候,使用帶指數衰減的學習率設置、使用正則化來避免過擬合、使用滑動平均模型來使得最終的模型更加健壯。

程序將計算神經網絡前向傳播的部分單獨定義一個函數inference,訓練部分定義一個train函數,再定義一個主函數main。

完整程序:

#!/usr/bin/env python3 # -*- coding: utf-8 -*- """ Created on Thu May 25 08:56:30 2017  @author: marsjhao """  import tensorflow as tf from tensorflow.examples.tutorials.mnist import input_data  INPUT_NODE = 784 # 輸入節點數 OUTPUT_NODE = 10 # 輸出節點數 LAYER1_NODE = 500 # 隱含層節點數 BATCH_SIZE = 100 LEARNING_RETE_BASE = 0.8 # 基學習率 LEARNING_RETE_DECAY = 0.99 # 學習率的衰減率 REGULARIZATION_RATE = 0.0001 # 正則化項的權重系數 TRAINING_STEPS = 10000 # 迭代訓練次數 MOVING_AVERAGE_DECAY = 0.99 # 滑動平均的衰減系數  # 傳入神經網絡的權重和偏置,計算神經網絡前向傳播的結果 def inference(input_tensor, avg_class, weights1, biases1, weights2, biases2):   # 判斷是否傳入ExponentialMovingAverage類對象   if avg_class == None:     layer1 = tf.nn.relu(tf.matmul(input_tensor, weights1) + biases1)     return tf.matmul(layer1, weights2) + biases2   else:     layer1 = tf.nn.relu(tf.matmul(input_tensor, avg_class.average(weights1))                    + avg_class.average(biases1))     return tf.matmul(layer1, avg_class.average(weights2))/              + avg_class.average(biases2)  # 神經網絡模型的訓練過程 def train(mnist):   x = tf.placeholder(tf.float32, [None,INPUT_NODE], name='x-input')   y_ = tf.placeholder(tf.float32, [None, OUTPUT_NODE], name='y-input')    # 定義神經網絡結構的參數   weights1 = tf.Variable(tf.truncated_normal([INPUT_NODE, LAYER1_NODE],                         stddev=0.1))   biases1 = tf.Variable(tf.constant(0.1, shape=[LAYER1_NODE]))   weights2 = tf.Variable(tf.truncated_normal([LAYER1_NODE, OUTPUT_NODE],                         stddev=0.1))   biases2 = tf.Variable(tf.constant(0.1, shape=[OUTPUT_NODE]))    # 計算非滑動平均模型下的參數的前向傳播的結果   y = inference(x, None, weights1, biases1, weights2, biases2)      global_step = tf.Variable(0, trainable=False) # 定義存儲當前迭代訓練輪數的變量    # 定義ExponentialMovingAverage類對象   variable_averages = tf.train.ExponentialMovingAverage(             MOVING_AVERAGE_DECAY, global_step) # 傳入當前迭代輪數參數   # 定義對所有可訓練變量trainable_variables進行更新滑動平均值的操作op   variables_averages_op = variable_averages.apply(tf.trainable_variables())    # 計算滑動模型下的參數的前向傳播的結果   average_y = inference(x, variable_averages, weights1, biases1, weights2, biases2)    # 定義交叉熵損失值   cross_entropy = tf.nn.sparse_softmax_cross_entropy_with_logits(           logits=y, labels=tf.argmax(y_, 1))   cross_entropy_mean = tf.reduce_mean(cross_entropy)   # 定義L2正則化器并對weights1和weights2正則化   regularizer = tf.contrib.layers.l2_regularizer(REGULARIZATION_RATE)   regularization = regularizer(weights1) + regularizer(weights2)   loss = cross_entropy_mean + regularization # 總損失值    # 定義指數衰減學習率   learning_rate = tf.train.exponential_decay(LEARNING_RETE_BASE, global_step,           mnist.train.num_examples / BATCH_SIZE, LEARNING_RETE_DECAY)   # 定義梯度下降操作op,global_step參數可實現自加1運算   train_step = tf.train.GradientDescentOptimizer(learning_rate)/              .minimize(loss, global_step=global_step)   # 組合兩個操作op   train_op = tf.group(train_step, variables_averages_op)   '''''   # 與tf.group()等價的語句   with tf.control_dependencies([train_step, variables_averages_op]):     train_op = tf.no_op(name='train')   '''   # 定義準確率   # 在最終預測的時候,神經網絡的輸出采用的是經過滑動平均的前向傳播計算結果   correct_prediction = tf.equal(tf.argmax(average_y, 1), tf.argmax(y_, 1))   accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))    # 初始化回話sess并開始迭代訓練   with tf.Session() as sess:     sess.run(tf.global_variables_initializer())     # 驗證集待喂入數據     validate_feed = {x: mnist.validation.images, y_: mnist.validation.labels}     # 測試集待喂入數據     test_feed = {x: mnist.test.images, y_: mnist.test.labels}     for i in range(TRAINING_STEPS):       if i % 1000 == 0:         validate_acc = sess.run(accuracy, feed_dict=validate_feed)         print('After %d training steps, validation accuracy'            ' using average model is %f' % (i, validate_acc))       xs, ys = mnist.train.next_batch(BATCH_SIZE)       sess.run(train_op, feed_dict={x: xs, y_:ys})      test_acc = sess.run(accuracy, feed_dict=test_feed)     print('After %d training steps, test accuracy'        ' using average model is %f' % (TRAINING_STEPS, test_acc))  # 主函數 def main(argv=None):   mnist = input_data.read_data_sets("MNIST_data", one_hot=True)   train(mnist)  # 當前的python文件是shell文件執行的入口文件,而非當做import的python module。 if __name__ == '__main__': # 在模塊內部執行   tf.app.run() # 調用main函數并傳入所需的參數list 

二、分析與改進設計

1. 程序分析改進

第一,計算前向傳播的函數inference中需要將所有的變量以參數的形式傳入函數,當神經網絡結構變得更加復雜、參數更多的時候,程序的可讀性將變得非常差。

第二,在程序退出時,訓練好的模型就無法再利用,且大型神經網絡的訓練時間都比較長,在訓練過程中需要每隔一段時間保存一次模型訓練的中間結果,這樣如果在訓練過程中程序死機,死機前的最新的模型參數仍能保留,杜絕了時間和資源的浪費。

第三,將訓練和測試分成兩個獨立的程序,將訓練和測試都會用到的前向傳播的過程抽象成單獨的庫函數。這樣就保證了在訓練和預測兩個過程中所調用的前向傳播計算程序是一致的。

2. 改進后程序設計

mnist_inference.py

該文件中定義了神經網絡的前向傳播過程,其中的多次用到的weights定義過程又單獨定義成函數。

通過tf.get_variable函數來獲取變量,在神經網絡訓練時創建這些變量,在測試時會通過保存的模型加載這些變量的取值,而且可以在變量加載時將滑動平均值重命名。所以可以直接通過同樣的名字在訓練時使用變量自身,在測試時使用變量的滑動平均值。

mnist_train.py

該程序給出了神經網絡的完整訓練過程。

mnist_eval.py

在滑動平均模型上做測試。

通過tf.train.get_checkpoint_state(mnist_train.MODEL_SAVE_PATH)獲取最新模型的文件名,實際是獲取checkpoint文件的所有內容。

三、TensorFlow最佳實踐樣例

mnist_inference.py

import tensorflow as tf  INPUT_NODE = 784 OUTPUT_NODE = 10 LAYER1_NODE = 500  def get_weight_variable(shape, regularizer):   weights = tf.get_variable("weights", shape,          initializer=tf.truncated_normal_initializer(stddev=0.1))   if regularizer != None:     # 將權重參數的正則化項加入至損失集合     tf.add_to_collection('losses', regularizer(weights))   return weights  def inference(input_tensor, regularizer):   with tf.variable_scope('layer1'):     weights = get_weight_variable([INPUT_NODE, LAYER1_NODE], regularizer)     biases = tf.get_variable("biases", [LAYER1_NODE],                  initializer=tf.constant_initializer(0.0))     layer1 = tf.nn.relu(tf.matmul(input_tensor, weights) + biases)    with tf.variable_scope('layer2'):     weights = get_weight_variable([LAYER1_NODE, OUTPUT_NODE], regularizer)     biases = tf.get_variable("biases", [OUTPUT_NODE],                  initializer=tf.constant_initializer(0.0))     layer2 = tf.matmul(layer1, weights) + biases    return layer2 

mnist_train.py

import os import tensorflow as tf from tensorflow.examples.tutorials.mnist import input_data import mnist_inference  BATCH_SIZE = 100 LEARNING_RATE_BASE = 0.8 LEARNING_RATE_DECAY = 0.99 REGULARIZATION_RATE = 0.0001 TRAINING_STEPS = 10000 MOVING_AVERAGE_DECAY = 0.99  MODEL_SAVE_PATH = "Model_Folder/" MODEL_NAME = "model.ckpt"  def train(mnist):   # 定義輸入placeholder   x = tf.placeholder(tf.float32, [None, mnist_inference.INPUT_NODE],             name='x-input')   y_ = tf.placeholder(tf.float32, [None, mnist_inference.OUTPUT_NODE],             name='y-input')   # 定義正則化器及計算前向過程輸出   regularizer = tf.contrib.layers.l2_regularizer(REGULARIZATION_RATE)   y = mnist_inference.inference(x, regularizer)   # 定義當前訓練輪數及滑動平均模型   global_step = tf.Variable(0, trainable=False)   variable_averages = tf.train.ExponentialMovingAverage(MOVING_AVERAGE_DECAY,                              global_step)   variables_averages_op = variable_averages.apply(tf.trainable_variables())   # 定義損失函數   cross_entropy = tf.nn.sparse_softmax_cross_entropy_with_logits(logits=y,                           labels=tf.argmax(y_, 1))   cross_entropy_mean = tf.reduce_mean(cross_entropy)   loss = cross_entropy_mean + tf.add_n(tf.get_collection('losses'))   # 定義指數衰減學習率   learning_rate = tf.train.exponential_decay(LEARNING_RATE_BASE, global_step,           mnist.train.num_examples / BATCH_SIZE, LEARNING_RATE_DECAY)   # 定義訓練操作,包括模型訓練及滑動模型操作   train_step = tf.train.GradientDescentOptimizer(learning_rate)/           .minimize(loss, global_step=global_step)   train_op = tf.group(train_step, variables_averages_op)   # 定義Saver類對象,保存模型,TensorFlow持久化類   saver = tf.train.Saver()    # 定義會話,啟動訓練過程   with tf.Session() as sess:     tf.global_variables_initializer().run()      for i in range(TRAINING_STEPS):       xs, ys = mnist.train.next_batch(BATCH_SIZE)       _, loss_value, step = sess.run([train_op, loss, global_step],                       feed_dict={x: xs, y_: ys})       if i % 1000 == 0:         print("After %d training step(s), loss on training batch is %g."/             % (step, loss_value))         # save方法的global_step參數可以讓每個被保存的模型的文件名末尾加上當前訓練輪數         saver.save(sess, os.path.join(MODEL_SAVE_PATH, MODEL_NAME),               global_step=global_step)  def main(argv=None):   mnist = input_data.read_data_sets("MNIST_data", one_hot=True)   train(mnist)  if __name__ == '__main__':   tf.app.run() 

mnist_eval.py

import time import tensorflow as tf from tensorflow.examples.tutorials.mnist import input_data import mnist_inference import mnist_train  EVAL_INTERVAL_SECS = 10  def evaluate(mnist):   with tf.Graph().as_default() as g:     # 定義輸入placeholder     x = tf.placeholder(tf.float32, [None, mnist_inference.INPUT_NODE],               name='x-input')     y_ = tf.placeholder(tf.float32, [None, mnist_inference.OUTPUT_NODE],               name='y-input')     # 定義feed字典     validate_feed = {x: mnist.validation.images, y_: mnist.validation.labels}     # 測試時不加參數正則化損失     y = mnist_inference.inference(x, None)     # 計算正確率     correct_prediction = tf.equal(tf.argmax(y, 1), tf.argmax(y_, 1))     accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))     # 加載滑動平均模型下的參數值     variable_averages = tf.train.ExponentialMovingAverage(                    mnist_train.MOVING_AVERAGE_DECAY)     saver = tf.train.Saver(variable_averages.variables_to_restore())      # 每隔EVAL_INTERVAL_SECS秒啟動一次會話     while True:       with tf.Session() as sess:         ckpt = tf.train.get_checkpoint_state(mnist_train.MODEL_SAVE_PATH)         if ckpt and ckpt.model_checkpoint_path:           saver.restore(sess, ckpt.model_checkpoint_path)           # 取checkpoint文件中的當前迭代輪數global_step           global_step = ckpt.model_checkpoint_path/                    .split('/')[-1].split('-')[-1]           accuracy_score = sess.run(accuracy, feed_dict=validate_feed)           print("After %s training step(s), validation accuracy = %g"/              % (global_step, accuracy_score))          else:           print('No checkpoint file found')           return       time.sleep(EVAL_INTERVAL_SECS)  def main(argv=None):   mnist = input_data.read_data_sets("MNIST_data", one_hot=True)   evaluate(mnist)  if __name__ == '__main__':   tf.app.run() 

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