title: celery - chaining groups and subtasks. -> out of order execution
body: <p>When I have something like the following</p>
  
  <pre><code>group1 = group(task1.si(), task1.si(), task1.si())
  group2 = group(task2.si(), task2.si(), task2.si())
  
  workflow = chain(group1, group2, task3.si())
  </code></pre>
  
  <p>The intuitive interpretation is that task3 should only execute after all tasks in group 2 have finished.  </p>
  
  <p>In reality, task 3 executes while group1 has started but hasn't completed yet.</p>
  
  <p>What am i doing wrong?</p>