机器学习-简单softmax模型的Tensorflow参数值不变

Question

受MNIST示例的启发，我正试图在图像数据上使用tensorflow来构建softmax模型。当我试图训练模型时，我发现损失并没有减少。我还看到，在第一次迭代之后，参数(W，b(值没有变化。我是否需要在每次迭代后显式更新我的参数值？

代码：-

######### Model Graph ###################
with tf.device('/cpu:0'):
x = tf.placeholder(tf.float32,shape = [None, IMAGE_HEIGHT, IMAGE_WIDTH, 3])
y_ = tf.placeholder(tf.float32,shape = [None,35])
########### Weight for each softmax sigmod function##############
initialW = tf.truncated_normal([IMAGE_HEIGHT*IMAGE_WIDTH*3, 35], stddev=0.1)
W = tf.Variable(initialW,trainable=True);
b = tf.Variable(tf.zeros([35]),trainable=True)

x_flat = tf.reshape(x, [-1,IMAGE_HEIGHT*IMAGE_WIDTH*3])
y=tf.nn.softmax(tf.matmul(x_flat,W)+b)
cross_entropy = tf.reduce_mean(-tf.reduce_sum(y_*tf.log(y+1e-10),reduction_indices=[1]))
cross_entropy = tf.Print(cross_entropy, [cross_entropy], "cost") #print to the console tensorflow
#train_step = tf.train.GradientDescentOptimizer(0.5).minimize(cross_entropy)
train_step = tf.train.AdamOptimizer(0.1).minimize(cross_entropy)
#### Model evaluation ######### Evaluating model
is_predicted_correctly = tf.equal(tf.argmax(y,1), tf.argmax(y_,1))
accuracy = tf.reduce_mean(tf.cast(is_predicted_correctly,tf.float32))
ops = tf.initialize_all_variables();

### Running graph ###
### Initialzing variable ####
config = tf.ConfigProto()
config.log_device_placement=True
sess = tf.Session(config=config)
sess.run(ops)
###Training####
for it in range(nIterations):
  labels, images = d.getNextBatch(nBatchSize)
  while(images is not None):
    sess.run(train_step, feed_dict = {x: images, y_ : labels})
    labels, images = d.getNextBatch(nBatchSize)

成本始终保持不变：

I tensorflow/core/kernels/logging_ops.cc:79] cost[22.211819]
I tensorflow/core/kernels/logging_ops.cc:79] cost[22.095526]
I tensorflow/core/kernels/logging_ops.cc:79] cost[22.676987]
I tensorflow/core/kernels/logging_ops.cc:79] cost[22.563032]

更新：批量代码

def getNextBatch(self,cnt):
    if(self.dataSet is None):
        return None, None;
    if(self.curr>=len(self.dataSet)):
        return None, None
    end = self.curr+cnt;
    if(end>len(self.dataSet)):
        end = len(self.dataSet)
    batchData = self.dataSet[self.curr:end]
    labelRaw = [];
    images = [];
    for dataPoint in batchData:
        try:
            image = self.getImageFromPath(dataPoint['image']);
            if(not self.isSizeCorrect(image)):
                print("Wrong image shape:"+str(image.shape));
                raise ValueError("Wrong image shape");
            labelRaw.append(dataPoint['label']);
            images.append(image);
        except (OSError, ValueError):
            k=0;
    labels = self.onEnc.transform((self.lEnc.transform(labelRaw)).reshape(-1,1))
    self.curr = end
    return labels, np.array(images)
def getImageFromPath(self,imageFile):
    img = misc.imread(imageFile)
    resizedImg = misc.imresize(img,(IMAGE_HEIGHT,IMAGE_WIDTH))
    return resizedImg;

Answer 1

我终于能够解决我的问题了。问题是，我的特征和权重的乘积很大(以千分之十为单位(，导致soft-max中的指数值(想象e^30000(膨胀。

正因为如此，我的梯度始终为零，因此参数没有更新。

我尝试了以下方法来解决这个问题：-

- Normalized my image data(pixel values from 0-to-255 to 0-to-1)
- Initialized parameter vectors with very small values around 10e-3
- Reduced the learning rate of my optimization algorithm. 

这会导致指数较小且梯度值为非零。终于能够训练模型了。