title: Python multiprocessing loop stops always for certain threads
body: <p>I want to accelarte data loading and interpolation by using multithreading. So far my code seems to work. The only problem is that it always runs up to the given number of procs.
  Say my files are 100. If I run the script with 24 cores only 24 files are being loaded, but I need to keep going on up to 100. What am I missing here?</p>
  
  <pre><code>        print ('Number of overall VTK Files: ' + str(len(vtkArray)))
      def dataoperator(N,i,vtkArray,field):
          #for i in range(N-100,N):
          print ("Loading data " +str(vtkArray[i,1] + ' index :' + str(i)))
          points, cells, point_data, cell_data, field_data = meshio.read(ID +str(vtkArray[i,1]))
          x,y,z=np.transpose(points)
          print ("\t Interpolating data " +str(vtkArray[i,1] + ' index :' + str(i)))
          from scipy.interpolate import griddata
          if (scalar=='p'):
              p = np.transpose(point_data['p'])
              pi= griddata((x, y, z), p, (xx, yy, zz), method='nearest')
              field[i,:,:,:]= pi
          else:
              u,v,w = np.transpose(point_data['subtract(U,U_bf)'])
              if (scalar=='u'):
                  ui= griddata((x, y, z), u, (xx, yy, zz), method='nearest')
                  field[i,:,:,:]= ui
              elif (scalar=='v'):
                  vi= griddata((x, y, z), v, (xx, yy, zz), method='nearest')
                  field[i,:,:,:]= vi
              else:
                  wi = griddata((x, y, z), w, (xx, yy, zz), method='nearest')
                  field[i,:,:,:]= wi
          del points, cells, point_data, cell_data, field_data
  
  
      import multiprocessing
      jobs = []
      for i in range(0, procs):
          process = multiprocessing.Process(target=dataoperator,args=(N, i, vtkArray,field))
          jobs.append(process)
  
          # Start the processes 
      for j in jobs:
          j.start()
  
          # Ensure all of the processes have finished
      for j in jobs:
          j.join()
  </code></pre>