Doubts about how implementing asynchronous timeouts through a heap

Josiah Carlson josiah.carlson at gmail.com
Sat Jul 12 21:16:54 CEST 2008


On Jul 9, 4:13 am, "Giampaolo Rodola'" <gne... at gmail.com> wrote:
> Hi,
> I'm trying to implement an asynchronous scheduler for asyncore to call
> functions at a later time without blocking the main loop.
> The logic behind it consists in:
>
> - adding the scheduled functions into a heapified list
> - calling a "scheduler" function at every loop which checks the
> scheduled functions due to expire soonest
>
> Note that, by using a heap, the first element of the list is always
> supposed to be the one with the lower timeout.
> Here's the code I wrote:
>
> <--- snippet --->
> import heapq
> import time
> import sys
>
> delayed_map = []
>
> class delayed_call:
>     """Calls a function at a later time.
>
>     The instance returned is an object that can be used to cancel the
>     scheduled call, by calling its cancel() method.
>     It also may be rescheduled by calling delay() or reset()} methods.
>     """
>
>     def __init__(self, delay, target, *args, **kwargs):
>         """
>         - delay: the number of seconds to wait
>         - target: the callable object to call later
>         - args: the arguments to call it with
>         - kwargs: the keyword arguments to call it with
>         """
>         assert callable(target), "%s is not callable" %target
>         assert sys.maxint >= delay >= 0, "%s is not greater than or
> equal " \
>                                            "to 0 seconds" % (delay)
>         self.__delay = delay
>         self.__target = target
>         self.__args = args
>         self.__kwargs = kwargs
>         # seconds from the epoch at which to call the function
>         self.timeout = time.time() + self.__delay
>         self.cancelled = False
>         heapq.heappush(delayed_map, self)
>
>     def __le__(self, other):
>         return self.timeout <= other.timeout
>
>     def active(self):
>         """Return True if this scheduler has not been cancelled."""
>         return not self.cancelled
>
>     def call(self):
>         """Call this scheduled function."""
>         self.__target(*self.__args, **self.__kwargs)
>
>     def reset(self):
>         """Reschedule this call resetting the current countdown."""
>         assert not self.cancelled, "Already cancelled"
>         self.timeout = time.time() + self.__delay
>         if delayed_map[0] is self:
>             heapq.heapify(delayed_map)
>
>     def delay(self, seconds):
>         """Reschedule this call for a later time."""
>         assert not self.cancelled, "Already cancelled."
>         assert sys.maxint >= seconds >= 0, "%s is not greater than or
> equal " \
>                                            "to 0 seconds" %(seconds)
>         self.__delay = seconds
>         self.reset()
>
>     def cancel(self):
>         """Unschedule this call."""
>         assert not self.cancelled, "Already cancelled"
>         del self.__target, self.__args, self.__kwargs
>         if self in delayed_map:
>             if delayed_map[0] is self:
>                 delayed_map.remove(self)
>                 heapq.heapify(delayed_map)
>             else:
>                 delayed_map.remove(self)
>         self.cancelled = True
>
> def fun(arg):
>     print arg
>
> a = delayed_call(0.6, fun, '0.6')
> b = delayed_call(0.5, fun, '0.5')
> c = delayed_call(0.4, fun, '0.4')
> d = delayed_call(0.3, fun, '0.3')
> e = delayed_call(0.2, fun, '0.2')
> f = delayed_call(0.1, fun, '0.1')
>
> while delayed_map:
>     now = time.time()
>     while delayed_map and now >= delayed_map[0].timeout:
>         delayed = heapq.heappop(delayed_map)
>         try:
>             delayed.call()
>         finally:
>             if not delayed.cancelled:
>                 delayed.cancel()
>     time.sleep(0.01)
> </--- snippet --->
>
> Here comes the questions.
> Since that the timeouts of the scheduled functions contained in the
> list can change when I reset() or cancel() them I don't know exactly
> *when* the list needs to be heapified().
> By doing some tests I came to the conclusion that I need the heapify()
> the list only when the function I reset() or cancel() is the *first of
> the list* but I'm not absolutely sure about it.
> When do you think it would be necessary calling heapify()?
> I wrote a short test suite which tests the code above and I didn't
> notice strange behaviors but since that I don't know much about the
> logic behind heaps I'd need some help.
> Thanks a lot in advance.
>
> --- Giampaolohttp://code.google.com/p/pyftpdlib/

I dug through my old pair heap implementation, did a little hacking on
heapq, and wrote a task scheduler system that plugs in to asyncore.

To schedule a task, you use:
task = asyncore.schedule_task(schedule, delay, callable, *args,
**kwargs)

Once you have that task object, you can then use:
asyncore.reschedule_task(schedule, delay, task)
asyncore.abs_reschedule_task(schedule, time, task)
... to reschedule the task into the future (or past).

You can also:
asyncore.delete_task(schedule, task)
... to completely remove the task from the scheduler.

Each one of these operations are O(logn), where n is the number of
tasks currently known to the scheduler.

To accommodate the new scheduler, asyncore.loop() now has the
following call signature.
def loop(timeout=30.0, use_poll=False, map=None, count=None,
schedule=None, use_schedule=False):

To try to help prevent poll_fcn() starvation (in the case of long-
running scheduled tasks), the task window execution is set just prior
to the poll_fun() call to be now + .01 seconds.  That is, window is
set, poll_fun() is called (which should handle current I/O
operations), and all tasks that are to be completed prior to the end
of the task window (now+.01 seconds, set prior to the poll_fun()
call).

Asyncore objects will not gain a set_scheduler() method, nor will they
gain a schedule keyword argument on instantiation.  Why?  Because the
scheduler is not required for socket I/O to work properly.  If you
want to use the scheduler from your own subclasses,
asyncore.<schedule_fcn>(asyncore.scheduled_tasks, ...) should be
sufficient.

This scheduler can be easily plugged into other systems, and it's
likely that I'll add it as an interactive scheduler to sched.py, put
the pair heap implementation into collections, and call it good.

 - Josiah



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