Tips on working with Theano

Debugging Theano code is notoriously hard. Perhaps the main reason is because the python code you just wrote is not executed. Instead, your python expressions are used to build a graph of what you want to compute. The graph is compiled into CPU or GPU code when you use something like theano.function to create a "regular" python function from your graph. Finally the compiled code is excuted only when you call the "regular" function, at which point, all the intermediate steps in the computation are hidden from view. As a result you can't just code and experiment with your code to better understand how things work.

Luckily there is a workaround that allows you to do just that. Somewhere at the start of the code enter something like:

import theano
theano.config.compute_test_value = 'warn'

if you want the above code to run only while debugging you can use the following if statement:

import sys
debug = sys.gettrace() is not None
if debug:

next, whenever you create a Theano variable, assign to it some value that will be used for debugging. For example:

import numpy
x = theano.tensor.matrix('features')
x.tag.test_value = numpy.random.rand(3, 2)

The value you just assigned for debugging can always be examined using x.tag.test_value and what is nice is that any expression that uses this variable also has its own test_value that is computed on the fly, as you would have expected from an interactive python code.

y = x*2
print y.tag.test_value

The expression can be broken into as many lines of code as you want as long as all the variables it is made off have their test_value defined. If a variable has a predefined value to it, set by Theano, then there is no need to set something using test_value

b = theano.tensor.ones((3,2))
c = theano.shared(np.ones((3,2)))
print (y*b + c).tag.test_value

In addition to exploring the intermediate results of your computation you will also get an excpetion whenever a test_value can not be computed immediately at the spot where the offending code was called. This usually happens to me because of misunderstanding of how Theano works.

I've found that it is important to use test values that are as realistic as possible. Use the same input values you expect to see when the final "regular" compiled function is going to be used.

However, if you want to speed things up while debugging you should start as small as possible. For example, use small number of hidden units.

Another trick that may help is using CPU at first:

export THEANO_FLAGS=device=cpu

Finally, if everything else fails, read the entire error message Theano throws at you, usually it is very cryptic but it can contain helpful hints.

You can even force some of the prints to appear even without an exception

theano.printing.debugprint(my_theano_variable, print_type=True)

I found that in many cases the GPU seats idle because the CPU is too slow feeding it with data. You can thread your CPU code etc but an easier solution will be to use the old code as is and run several experiments in parallel. For example, try different hyper parameter. Just open another shell and run a different experiment on the same GPU, Theano is fine with that.

If you put “profile=True” into THEANO_FLAGS, it will analyse your program, showing a breakdown of how much is spent on each operation. Very handy for finding bottlenecks.

Theano tutorials Summer School 2015