python script is slowly after use multiprocessing

[MRAB]

On 2017-07-03 20:47, Xristos Xristoou wrote:
> i have create an image processing python function.
> 
> my system have 4 cores + 4 threads.
> 
> i want to use multiprocessing to speed up my function,but anytime to use multiprocessing packages my function is not faster and is 1 minute slowly. any idea why ?first time use multiprocessing packages.
> 
> main function :
> 
> if __name__ == '__main__':
>      in_path="C:/Users/username/Desktop/in.tif"
>      out_path="C:/Users/username/Desktop/out.tif"
>      myfun(in_path, out_path)
> time=3.4 minutes
> 
> with multiprocessing map :
> 
> if __name__ == '__main__':
>      p = Pool(processes=4)
>      in_path="C:/Users/username/Desktop/in.tif"
>      out_path="C:/Users/username/Desktop/out.tif"
>      result = p.map(myfun(in_path,out_path))
> time=4.4 minutes
> 
> if __name__ == '__main__':
>      pool = multiprocessing.Pool(4)
>      in_path="C:/Users/username/Desktop/in.tif"
>      out_path="C:/Users/username/Desktop/out.tif"
>      pool.apply_async(myfun, args=(in_path,out_path,))
>      pool.close()
>      pool.join()
> time=4.5 minutes
> 
It looks like you have one function performing one task. It's not 
splitting the task into multiple parts that can be run in parallel on 
multiple cores.