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.