What is the difference between steps and epochs in TensorFlow?

In most of the models, there is a steps parameter indicating the number of steps to run over data. But yet I see in most practical usage, we also execute the fit function N epochs.

What is the difference between running 1000 steps with 1 epoch and running 100 steps with 10 epoch? Which one is better in practice? Any logic changes between consecutive epochs? Data shuffling?

A training step is one gradient update. In one step batch_size examples are processed.

An epoch consists of one full cycle through the training data. This is usually many steps. As an example, if you have 2,000 images and use a batch size of 10 an epoch consists of:

2,000 images / (10 images / step) = 200 steps.

If you choose your training image randomly (and independently) in each step, you normally do not call it epoch. [This is where my answer differs from the previous one. Also see my comment.]

An epoch usually means one iteration over all of the training data. For instance if you have 20,000 images and a batch size of 100 then the epoch should contain 20,000 / 100 = 200 steps. However I usually just set a fixed number of steps like 1000 per epoch even though I have a much larger data set. At the end of the epoch I check the average cost and if it improved I save a checkpoint. There is no difference between steps from one epoch to another. I just treat them as checkpoints.

People often shuffle around the data set between epochs. I prefer to use the random.sample function to choose the data to process in my epochs. So say I want to do 1000 steps with a batch size of 32. I will just randomly pick 32,000 samples from the pool of training data.

As I am currently experimenting with the tf.estimator API I would like to add my dewy findings here, too. I don't know yet if the usage of steps and epochs parameters is consistent throughout TensorFlow and therefore I am just relating to tf.estimator (specifically tf.estimator.LinearRegressor) for now.

Training steps defined by num_epochs: steps not explicitly defined

estimator = tf.estimator.LinearRegressor(feature_columns=ft_cols)
train_input =  tf.estimator.inputs.numpy_input_fn({'x':x_train},y_train,batch_size=4,num_epochs=1,shuffle=True)
estimator.train(input_fn=train_input)

Comment: I have set num_epochs=1 for the training input and the doc entry for numpy_input_fn tells me "num_epochs: Integer, number of epochs to iterate over data. If None will run forever.". With num_epochs=1 in the above example the training runs exactly x_train.size/batch_size times/steps (in my case this was 175000 steps as x_train had a size of 700000 and batch_size was 4).

Training steps defined by num_epochs: steps explicitly defined higher than number of steps implicitly defined by num_epochs=1

estimator = tf.estimator.LinearRegressor(feature_columns=ft_cols)
train_input =  tf.estimator.inputs.numpy_input_fn({'x':x_train},y_train,batch_size=4,num_epochs=1,shuffle=True)
estimator.train(input_fn=train_input, steps=200000)

Comment: num_epochs=1 in my case would mean 175000 steps (x_train.size/batch_size with x_train.size=700,000 and batch_size=4) and this is exactly the number of steps estimator.train albeit the steps parameter was set to 200,000 estimator.train(input_fn=train_input, steps=200000).

Training steps defined by steps

estimator = tf.estimator.LinearRegressor(feature_columns=ft_cols)
train_input =  tf.estimator.inputs.numpy_input_fn({'x':x_train},y_train,batch_size=4,num_epochs=1,shuffle=True)
estimator.train(input_fn=train_input, steps=1000)

Comment: Although I have set num_epochs=1 when calling numpy_input_fnthe training stops after 1000 steps. This is because steps=1000 in estimator.train(input_fn=train_input, steps=1000) overwrites the num_epochs=1 in tf.estimator.inputs.numpy_input_fn({'x':x_train},y_train,batch_size=4,num_epochs=1,shuffle=True).

Conclusion: Whatever the parameters num_epochs for tf.estimator.inputs.numpy_input_fn and steps for estimator.train define, the lower bound determines the number of steps which will be run through.