Fix TensorFlow UserWarning: Converting sparse IndexedSlices to a dense Tensor of unknown shape – TensorFlow Tutorial

By | July 19, 2020

This UserWarning error is:

UserWarning: Converting sparse IndexedSlices to a dense Tensor of unknown shape. This may consume a large amount of memory.
“Converting sparse IndexedSlices to a dense Tensor of unknown shape. “

In this tutorial, we will introduce how to fix this UserWarning when you are using tensorflow to build a model.

Why does this UserWarning occur?

If you are using tf.gather() or tf.nn.embedding_lookup() to get a tensor, you may encounter it.

How to fix this UserWarning?

Some programmers may tell you to pass a tf.Variable to tf.gather() or tf.nn.embedding_lookup() to fix this warning.

For example:

To fix this problem, you should try to ensure that the params input to tf.gather() (or the params inputs to tf.nn.embedding_lookup()) is a tf.Variable.

https://stackoverflow.com/questions/35892412/tensorflow-dense-gradient-explanation

However, this way can not fix it.

As to us, we have used tf.nn.embedding_lookup() in a tf.while_loop().

The code is below:

        def _g_recurrence(i, x_t, h_tm1, gen_o):
            h_t = self.g_recurrent_unit(x_t, h_tm1)
            o_t = self.g_output_unit(h_t)  # batch x 200
            
            gen_o = gen_o.write(i, o_t)
            i_next = tf.where(tf.less(i, self.time_step-1), i+1, self.time_step-1)
            x_t_next = tf.nn.embedding_lookup(self.inputs,i_next) #batch x emb_dim
           
            return i+1, x_t_next, h_t, gen_o

We will get a tensor from self.inputs by i_next, i_next is a tf.Variable. However, we also get this UserWarning.

Converting sparse IndexedSlices to a dense Tensor of unknown shape

A good way to fix this UserWarning is to convert self.inputs to a TensorArray, then we can read tensor by TensorArray.read() function.

To convert a tensor to tensorarray, you can read:

Best Practice to Convert a Tensor to TensorArray in TensorFlow

Here is an example:

        self.inputs_ta = tf.TensorArray(dtype=tf.float32, size=self.time_step ,
                                             dynamic_size=False, infer_shape=True)
        self.inputs_ta = self.inputs_ta.unstack(self.inputs)

Then we can fix _g_recurrence() function as following:

        def _g_recurrence(i, h_tm1, gen_o):
           
            #x_t = tf.nn.embedding_lookup(self.inputs,i) #batch x emb_dim
            x_t = self.inputs_ta.read(i)
            h_t = self.g_recurrent_unit(x_t, h_tm1)  
            o_t = self.g_output_unit(h_t) 
            
            gen_o = gen_o.write(i, o_t)
            return i+1, h_t, gen_o

Then we find this UserWarning disappeared.

fix Converting sparse IndexedSlices to a dense Tensor of unknown shape

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