txRDQ (Resizable Dispatch Queue) provides a Twisted class for a
controllable priority queue for running jobs. You can obtain the contents
of the queue and the dispatched pool while running.  Jobs can be canceled
or reprioritized.  The size of the underway job pool can be changed
dynamically.  Job dispatch can be paused and resumed. The queue can be
stopped gracefully, returning jobs that had not yet been dispatched.

See http://blogs.fluidinfo.com/terry/2011/06/27/a-resizable-dispatch-queue-for-twisted/ for some more detail.