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After reading this, I decided to transition my DQN code from the keras library to tf.keras library (code is located in this repo) and my original code used NCHW format, as it was faster with GPUs. As I need to run in CPUs also, I found out that it ran only with NHWC format on my tf version (1.12 with python 3.7, compiled with AVX2 flags).

I redesigned my code and it's working, but I stack in the NCHW format and then I have to transpose it every function call, which is costly.

My code

There is a 'frames' list sized 'self.nb_frames', which will hold the (10, 10) states. Then I expand_dims and transpose, returning the list in NHWC format.

example.py

#!/usr/bin/env python



import numpy as np



class Agent():



    def __init__(self):

        """Initialize the agent with given attributes."""

        self.frames = None

        self.nb_frames = 4



    def get_game_data(self, state):

        """Create a list with 4 frames and append/pop them each frame."""

        frame = state



        if self.frames is None:

            self.frames = [frame] * self.nb_frames

        else:

            self.frames.append(frame)

            self.frames.pop(0)



        # from (4, 10, 10) to (1, 4, 10, 10)

        expanded_frames = np.expand_dims(self.frames, 0)



        # From (1, 4, 10, 10) to (1, 10, 10, 4) | NCHW -> NHWC

        expanded_frames = np.transpose(expanded_frames, [0, 3, 2, 1])



        return expanded_frames





board_size = 10

state = np.zeros((board_size, board_size))

agent = Agent()

stacked_state = agent.get_game_data(state)

In order to verify how costly is to transpose, I've executed the code below for two conditions:

1. Without transposing (NCHW) = 5.775287926 s;


2. 
   

   Transposing (NHWC) = 7.381751397 s.

   
   
   

   from timeit import Timer
   
   
   
   t = Timer(lambda: agent.get_game_data(state))
   
   print (t.timeit(number = 1000000))
   
   

   
   

So transposing is responsible for 27% of the running time of the function get_game_data.

Is there a better option to create the list directly in the (10, 10, 4) format? Does my code follow best practices?