title: Python read image to numpy array with multiprocessing
body: <p>I am working on a python program which read a lot of images in batches (let's say 500 images) and store it in a numpy array.</p>
  
  <p>Now it's single thread, and IO is very fast, the part which take a lot of time is creating numpy array and doing something on it.</p>
  
  <p>By using multiprocessing module, I am able to read and create the array in other process. But I am having problem let the main thread access those data.</p>
  
  <p>I have tried:</p>
  
  <p>1: Using multiprocessing.queues:  Very slow, I believe it's the pickle and unpickle waste a lot of time. Pickling and unpickling a large numpy array take quite some time.</p>
  
  <p>2: Using Manager.list():  Faster than queues, but when try to access it in main thread, it 's still very slow. Even just iterate over the list and do nothing takes 2 seconds per item.
  I don't understand why it take so much time.</p>
  
  <p>Any suggestions ? Thanks.</p>