# When to use model.train() vs model.eval() in Pytoch?

I have a model that is used in a reinforcement learning algorithm for checkers, a la AlphaZero. Similar to that network, mine features batch normalization after each convolution layer. I am aware that this will cause different behavior/output when using .eval() vs .train()

However, I am unsure of when to use eval() vs train(). The model is used at two different points in the algorithm: First, the network is used to generate many games of self-play. Secondly, the network is trained using the positions of theses games, with the evaluation labels taken from the terminal value of the game (-1, 0, +1) and the 'improved policy' labels are taken to be the visit counts after the UCB-tree-search.

It seems to me that when the network is fully trained, I will use .eval(), as that should be 'what the network really thinks'. Therefore, for the games of self-play, I should also use .eval(). Ostensibly, this should result in stronger games of self-play and thus higher quality data. Finally, if I used .eval() in the self-play step I must also use it in the learning phase, otherwise if the network outputs are different the loss won't even be calculated using the actual outputs! I know that the network learns valuable information from .train(), as the batch norm layers learn about the mean/variance of the data universe.

As I type this, I am starting to suspect that I should be using .train() for both the self-play and learning phases. Still that seems wrong as the discrepancy in output between train() and eval() for a given position can be quite large.

I can see arguments to have the self-play phase use both .train() and .eval(), so I had a look at the implementation of facebook's ELF OpenGo and saw that they have the model in eval mode during self-play (see selfplay.py). I would do as they do because their software seems to work.

You can see from the PyTorch documentation that the eval and the train do the same thing. Although they don't explicitly mention it, the documentation is identical:

Sets the module in evaluation mode.

This has any effect only on certain modules. See documentations of particular modules for details of their behaviors in training/evaluation mode, if they are affected, e.g. Dropout, BatchNorm, etc.

Additional ideas from this PyTorch forum:

Yes, they are the same. By default all the modules are initialized to train mode (self.training = True). Also be aware that some layers have different behavior during train/and evaluation (like BatchNorm, Dropout) so setting it matters. Also as a rule of thumb for programming in general, try to explicitly state your intent and set model.train() and model.eval() when necessary.

Regarding the situations of self-play and then evaluation, I personally would start by doing self-play in .eval() mode in order to have the highest fidelity between self-play and the final execution mode (i.e. a real game). The discrepancy that you speak of should converge as the model does, but I agree it might be problematically large to begin with.

One idea would be to use a warm start... run in .train() mode for a given number of iterations (practice games, or just rounds of self-play), then phase this out as the model hopefully begins to converge.

• I am aware of the behaviour of eval() and train(). I am asking which would be the more appropriate choice in my particular algorithm Jun 27 '19 at 15:00
• @basket - sorry for the misunderstanding. Please see my update for some ideas/opinions. Jun 27 '19 at 15:18
• Good suggestion, I will try it. It is probably not a cardinal sin to never update the BN layers, but I can still take the random network and run all my data through it and save it as version 0.5 Jun 27 '19 at 15:45