Marcel Keller <mkel...@cs.au.dk> writes: >> 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.
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?
> Of course, we can avoid hacking the Twisted code by extending it
> within VIFF. Still, I'm in favor of the two-threaded solution because
> it's more elegant, doesn't have the danger of recursing too deep, and,
> in my opinion, it should be feasible.
I have not yet seen the two-threaded approach -- do you have a patch?
>>> 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?
>> 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.
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.
>> 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.
My goal is to violate as few Twisted docstrings as possible :-) And to
use the tools provided by Twisted as much as possible.
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.
> The principle is that not only the communication with doSelect() is
> done but also the callbacks with process_deferred_queue() are
> processed. Only the latter triggers further communication and ends the
> lifetime of deferreds, which frees memory.
>
> The following graph illustrates my hack:
>
> myIteration()
> / \
> doSelect() process_deferred_queue()
> | |
> communication callbacks
> | |
> stringReceived() mul() / open() etc.
> | |
> queue_deferred() myIteration()
> |
> ...
So the idea is that to avoid recursing in doSelect, the doSelect branch
of the call tree must terminate before any callbacks are actually
executed... that makes sense.
--
Martin Geisler
VIFF (Virtual Ideal Functionality Framework) brings easy and efficient
SMPC (Secure Multiparty Computation) to Python. See: http://viff.dk/.
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