title: Pool.map hangs after 4 iterations
body: <p>I'm new to Python and multiprocessing. I wanted to batch convert a bunch of files, so I figured I'd try out multiprocessing. The Pool and map() concepts seemed simple enough, but it doesn't seem to work. I have reduced it to a test program below, but the gist of it is: it will go through 4 iterations (per process in Pool) and hang after that.  Here's the test code:</p>
  
  <pre><code>import multiprocessing, logging
  import os
  import sys
  
  mpl = multiprocessing.log_to_stderr()
  mpl.setLevel(logging.INFO)
  
  def chill(t):
      cmd  = '/bin/sleep'
      args = (cmd,str(t))
      print &gt;&gt;sys.stderr, os.getpid(), args
      os.execv(cmd, args)
  
  if __name__ == "__main__" :
      times = [ 1 ] * 100
      pool = multiprocessing.Pool(1)  # change this for more processes
      pool.map(chill, times)
      pool.close()
      pool.join()
  </code></pre>
  
  <p>When I run it, it hangs after the 4th iteration. Increasing the number of processes just makes it hang at 4x the number of processes.  What's magical about this "4", and what am I doing wrong?</p>