Tensorflow Cifar10教程示例失败了

Question

I am currently trying to teach myself TensorFlow. After thorough reading and videos, I tried to re-create to example provided at https://www.tensorflow.org/versions/r0.12/tutorials/mnist/beginners/index.html#mnist-for-ml-beginners However, to not only copy&paste, I decided to make small alterations, to actually see whether I understand what I am doing, thus I decided to work with the CIFAR-10 dataset (small 32x32 rgb images).

The code skeleton is pretty much the basic skeleton, like it is presented in the tutorial:

# Imports
import tensorflow as tf
import numpy as np

###
### Open data files (dict)
###

def unpickle(file):
    import cPickle
    fo = open(file, 'rb')
    dict = cPickle.load(fo)
    fo.close()
    return dict

cifar10_test = unpickle('cifar-10-batches-py/test_batch')
cifar10_meta = unpickle('cifar-10-batches-py/batches.meta')
cifar10_batches = [unpickle('cifar-10-batches-py/data_batch_1'),
    unpickle('cifar-10-batches-py/data_batch_2'),
    unpickle('cifar-10-batches-py/data_batch_3'),
    unpickle('cifar-10-batches-py/data_batch_4'),
    unpickle('cifar-10-batches-py/data_batch_5')]

###
### Tensorflow Model
###
x = tf.placeholder("float", [None, 3072]) 
W = tf.Variable(tf.zeros([3072,10]))
b = tf.Variable(tf.zeros([10]))
y = tf.nn.softmax(tf.matmul(x,W) + b)
y_ = tf.placeholder("float", [None,10])

cross_entropy = -tf.reduce_sum(y_*tf.log(y))
train_step = tf.train.GradientDescentOptimizer(0.01).minimize(cross_entropy)

sess = tf.Session()
sess.run(tf.global_variables_initializer())

###
### Model training
###

for batch in cifar10_batches:
    # Convert labels to vector with zeros, but 1 at correct position
    batch['labels_vec'] = np.zeros((10000,10), dtype=float, order='C')
    for i in range(10000):
        batch['labels_vec'][i][batch['labels'][i]] = 1

    # Train in smaller sub-batches
    for i in range(3): # Breaks at first iteration, so no need to go on further
        start = i*100
        stop = start+100
        [_, cross_entropy_py] = sess.run([train_step, cross_entropy],
            feed_dict={x: batch['data'][start:stop],
            y_: batch['labels_vec'][start:stop]})
        print 'loss = %s' % cross_entropy_py
    break # Only first batch for now

This leaves me with the output:

loss = 230.259
loss = nan
loss = nan

No error is provided by the console. I tried searching for people with the same problem, however only found different questions of scenarios which resulted in "nan" values.

The only things I changed from the online tutorial: The originally used dataset had handwritten numbers with 28x28 pixels on greyscale, thus only 784 values, instead of 3072. However, I believe this should not fundamentally change much, as I also changed the dimensions of the placeholders.

Additionally, my label values were given as a list of numbers between 0 and 9. I changed this to be zero vectors, where the correct position is indicated with a 1. eg if it was 3, it would be replaced with [0 0 0 1 0 0 0 0 0 0]

Some hints on where I should aim my debugging would be helpful. I had a bigger stepsize of 0.1 for the GradientDescentOptimizer, but reduced that to 0.01 (the original value as used in the tutorial) after reading that a too big stepsize may result in the loss diverging to nan.

Thank you in advance.

Answer 1

Your loss is not numerically stable. You can use a loss that is already implemented for multiclass logistic regression instead of your loss: sigmoid_cross_entropy_with_logits . It was carefully designed to avoid numerical problems.