I think we would get the same result if we started a LoopingCall that
executes process_deferred_queue with an interval of, say, 100 ms:

  
http://twistedmatrix.com/documents/8.2.0/api/twisted.internet.task.LoopingCall.html

This should work since the runUntilCurrent method runs through the
waiting calls and will trigger our process_deferred_queue method.

And voilá -- no hacking of the Twisted source needed.
I'm not sure but LoopingCall._reschedule() looks more like it
schedules the calls at certain tick, not as soon as possible after the
interval is elapsed. This might not cost too much time but still
doesn't feel very elegant. Furthermore, setting the interval very low
leads to high CPU usage when waiting. Again, this is not too bad but
not elegant either. The same applies if using reactor.callLater()
directly.

A looping call is just a higher level wraper for doing

  def reschedule(func):
    func()
    reactor.callLater(interval, reschedule, func)

  reschedule(func)

It will execute the function when the (now + interval) time has been
reached and when the control flow returns to the reactor's event loop.
We probably wont need the extra logic in a looping call, so we can just
let the function reschedule itself like above.

That's what I meant with calling reactor.callLater() directly.

If we do this with an interval of 0, then the function will be called on
each iteration through the reactor's event loop -- just like your
loopCall I believe?

Not exactly because it also sets the timeout of the select call to 0
leading to 100% CPU usage while when we are waiting.

diff -r e2759515f57f viff/runtime.py
--- a/viff/runtime.py   Thu Mar 05 21:02:57 2009 +0100
+++ b/viff/runtime.py   Fri Mar 06 13:43:14 2009 +0100
@@ -306,6 +306,8 @@
                 deferred = deq.popleft()
                 if not deq:
                     del self.incoming_data[key]
+                # just queue
+                self.factory.runtime.queue_deferred(deferred)
Why is this done?
At this time, we shouldn't call the callbacks because we might recurse
into selectreactor.doSelect(). However, we want to know which
deferreds are ready so we can call deferred.callback() later.

Uhh, this sounds a bit dangerous -- do we know exactly which Deferreds
we can invoke callback on and which we cannot? As I remember, we invoke
callback in a couple of other locations, is that safe?

Yes, it is safe because the callback is called only once. When the data arrives, the Deferreds are paused, appended to the queue, and the callback is called. The Deferres in the queue are unpaused and removed in process_deferred_queue(). As far as I know you can pause and unpause Deferreds as you like.

If that doesn't matter, then I think this would be faster:

  queue, self.deferred_queue = self.deferred_queue, []
  map(Deferred.unpause, queue)

My idea is that looping over the list with map is faster than
repeatedly popping items from the beginning (an O(n) operation).
But map() still would need O(n) time because that is the nature of
calling a function n times, isn't it? Maybe the function calls are
optimized but the code in the function still is called n times.

Each pop(0) call is an O(n) operation, so we get O(n^2) here -- it is an
expensive way to loop through a list. And now that I look at it, using
map will still unpause the Deferreds in the order as you added them.

OK, I wasn't aware that pop(0) is O(n), but I still think that the
complexities should be added resulting in running time O(n) again. Using a linked list would be more reasonable, of course.

The difference is then that anything added to the queue as a result of
the unpause calls will be processed the next time the code is called.

Yes, and the Deferreds in the queue previously would wait. I considered it to be more safe if the Deferreds are processed in the order they arrive.

A question springs to my mind: calling

  reactor.runUntilCurrent()
  reactor.doIteration(0)

is the same as calling

  reactor.iterate(0)

and the documentation for that method says:

  [...] This method is not re-entrant: you must not call it recursively;
  in particular, you must not call it while the reactor is running.

How does your code ensure that we only call myIteration when we're
not in a call made by the reactor? And could we simply call
reactor.iterate instead?
We actually call it recursively but it should be reentrant if it's not
called from doIteration(). doIteration() is a the same as
select.doSelect(), which certainly is not reentrant. We however call
it from the loop call (process_deferred_queue()) after doIterate().

Calling reactor.iterate() is not enough because it doesn't call
process_deferred_queue().

So if we call reactor.iterate and then runtime.process_deferred_queue as

  reactor.callLater(0, reactor.iterate)
  reactor.callLater(0, runtime.process_deferred_queue)

we should be fine? They would then both run one after another just as
soon as the event loop is reached.

Isn't that more or less the same as your definition of reschedule() above? Again, i would say to callLater(0, ...) sets the timeout of the select call to 0 resulting in 100% CPU usage while we are waiting.

My goal is to violate as few Twisted docstrings as possible :-) And to
use the tools provided by Twisted as much as possible.

Sure, it always is wise to use libraries in the way they're meant to. I'm in favor of the two-thread solution because I think that it can be implemented without any changes to Twisted.

I would also like to hear what the Twisted guys have to say about
calling reactor.iterate like this, it would be nice if they could say
that it *is* infact safe to do it like this.

Jean-Paul Calderone wrote in his reply to my mail that there is a way to make reactor.run() return without stopping the reactor. Only then a call to reactor.iterate() is safe.
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