ValueError: Trying to share variable rnn/multi_rnn

Question

This it the code:

X = tf.placeholder(tf.float32, [batch_size, seq_len_1, 1], name='X')
labels = tf.placeholder(tf.float32, [None, alpha_size], name='labels')

rnn_cell = tf.contrib.rnn.BasicLSTMCell(512)
m_rnn_cell = tf.contrib.rnn.MultiRNNCell([rnn_cell] * 3, state_is_tuple=True)
pre_prediction, state = tf.nn.dynamic_rnn(m_rnn_cell, X, dtype=tf.float32)

This is full error:

ValueError: Trying to share variable rnn/multi_rnn_cell/cell_0/basic_lstm_cell/kernel, but specified shape (1024, 2048) and found shape (513, 2048).

I'm using a GPU version of tensorflow.

Answer 1

I guess it's because your RNN cells on each of your 3 layers share the same input and output shape.

On layer 1, the input dimension is 513 = 1(your x dimension) + 512(dimension of the hidden layer) for each timestamp per batch.

On layer 2 and 3, the input dimension is 1024 = 512(output from previous layer) + 512 (output from previous timestamp).

The way you stack up your MultiRNNCell probably implies that 3 cells share the same input and output shape.

I stack up MultiRNNCell by declaring two separate types of cells in order to prevent them from sharing input shape

rnn_cell1 = tf.contrib.rnn.BasicLSTMCell(512)
run_cell2 = tf.contrib.rnn.BasicLSTMCell(512)
stack_rnn = [rnn_cell1]
for i in range(1, 3):
    stack_rnn.append(rnn_cell2)
m_rnn_cell = tf.contrib.rnn.MultiRNNCell(stack_rnn, state_is_tuple = True)

Then I am able to train my data without this bug. I'm not sure whether my guess is correct, but it works for me. Hope it works for you.

Answer 2

I encountered a similar problem when I upgraded to v1.2 (tensorflow-gpu). Instead of using [rnn_cell]*3, I created 3 rnn_cells (stacked_rnn) by a loop (so that they don't share variables) and fed MultiRNNCell with stacked_rnn and the problem goes away. I'm not sure it is the right way to do it.

stacked_rnn = []
for iiLyr in range(3):
    stacked_rnn.append(tf.nn.rnn_cell.LSTMCell(num_units=512, state_is_tuple=True))
MultiLyr_cell = tf.nn.rnn_cell.MultiRNNCell(cells=stacked_rnn, state_is_tuple=True)

Answer 3

An official TensorFlow tutorial recommends this way of multiple LSTM network definition:

def lstm_cell():
  return tf.contrib.rnn.BasicLSTMCell(lstm_size)
stacked_lstm = tf.contrib.rnn.MultiRNNCell(
    [lstm_cell() for _ in range(number_of_layers)])

You can find it here: https://www.tensorflow.org/tutorials/recurrent

Actually it it almost the same approach that Wasi Ahmad and Maosi Chen suggested above but maybe in a little bit more elegant form.