Comment 9 for bug 1454873

Ryan's testing with TripleO reduced the peak memory usage after the update of a TripleO overcloud stack from ~540MB to ~133MB. That suggests that we found all the loops. The excessive memory usage before was likely due to all of the extra stacks loaded as a result of bug 1455589, with most of them remaining in memory for long enough to completely fragment Python's memory pools as a result of them not being garbage collected in time.