In fit(), how is the validation split computed?

If you set the validation_split argument in model.fit to e.g. 0.1, then the validation data used will be the last 10% of the data. If you set it to 0.25, it will be the last 25% of the data, etc. Note that the data isn’t shuffled before extracting the validation split, so the validation is literally just the last x% of samples in the input you passed.

The same validation set is used for all epochs (within the same call to fit).

Note that the validation_split option is only available if your data is passed as Numpy arrays (not tf.data.Datasets, which are not indexable).