Spent the first 4 or 5 hours figuring out what the heck went wrong with the dratted ping scanner (which if you will recall was supposed to be working fine on Friday, but which wound up hung for most of the weekend). What was going wrong was that I was doing something like this:

result = deferToThread( someOperation )
for value in result:
    doSomething(value)

The problem is that pytable creates lazy-execution cursors by default (which is also the underlying PyPgSQL's default).