Friday, May 29, 2009

Is Python really slow? A practical comparison with C++

The common perception is that Python's implementation is slow, but you can often write fast Python if you know how to profile your code effectively.

I have tried it. I have compared a hight cpu intensive algorithm, the training of a simple one hidden neural network. To do so, I have used my old C++ NeuralNetwork library (flayers) and an implementation in python with Numpy. I have wrote a simple neural net in python and optimize all loops with numpy as suggested in a profiling presentation saw in Pycon2009.

I have compare the training time of a simple fully connected NeuralNetwork will 100 hidden neurones for 10 iteration on letters dataset (cost function = mean square error).
Here is the time to do 10 iteration with flayers (c++):
./fexp / -h 100 -l 0.01 --oh -e 10
Optimization: Standard
Creating Connector [16|100] [inputs | hiddens]
Creating Connector [100|26] [hiddens | outputs]

real 0m11.187s
user 0m10.837s
sys 0m0.012s
Here is the time to do 10 iteration on the full letters dataset with python:

time ./ -e 10 --h 100 -f letters.dat -n
Creation of an NN <16:100:26>
real 85m48.646s
user 85m9.163s
sys 0m1.632s
Here is the time to do 10 iteration on the full letters dataset with python and numpy:time
./ -e 10 --h 100 -f letters.dat
Creation of an NN <16:100:26>
real 1m37.066s
user 1m36.026s
sys 0m0.100s

So if you do the math:
  • The numpy implementation is 60 time faster then a basic python implementation.
  • My C++ implementation is a little more then 10 time faster then my simply python numpy implementation.
Numpy implementation definitly worth it because it reduce the code and has a significant performance impact, the C++ might be required for extreme performance but the trade off of code complexity and time my not work it. Now that I have the choice, I will still use my C++ lib.

1 comment:

  1. great article.
    It would be better to let's know how long it takes you to
    develop the c++ version and the python , numpy version ,
    so we would know the the efficiency for programmer.

    Also can you modify your code so it can runs on cloud computation?
    how easy it would be to modify the python code than the c++ code?
    Also is it possible to use pyCUDA?