Python threading tutorial?

Jp Calderone exarkun at intarweb.us
Sun Jan 5 19:25:48 CET 2003


On Sun, Jan 05, 2003 at 10:03:38AM +0100, Gerrit Holl wrote:
> Dennis Lee Bieber schreef op zondag  5 januari om 01:30:38 +0000:
> > [snip]
> > On collision, that thread would have to message the other 
> > thread to do the interaction. Sounds like an awful lot of message 
> > Queues (see Queue.Queue) and maybe synchronization primitives (see 
> > threading.Semaphore, threading.Event, threading.Condition).
> 
> ...so I am currently not using the queues. Does that mean my application
> is not reliable, or is this a reliable solution? It seems to run ok...

  That's the problem with threads -- they seem to work for a while, even
when you're using them wrong.  

> 
> My source code:
> [big snip]
> 
> 		if isinstance(self.mx[newx][newy], self.__class__):
> 			raise Collision
> 
> 		self.mx[self.x][self.y] = self.mx.bg
> 		self.mx[newx][newy] = self
> 		self.x = newx
> 		self.y = newy
> 

   Above, you have a race condition.  If thread A and thread B are both
moving to the same matrix location and thread A runs the first two lines,
then thread B runs the whole block, when thread A resumes, it will believe
the destination to be unoccupied and happily trample all over thread B.  It
probably seems to work right because you haven't tested the system under a
high load yet.

  Jp

-- 
Lowery's Law:
        If it jams -- force it.  If it breaks, it needed replacing anyway.
--
 12:00am up 20 days, 9:46, 2 users, load average: 0.10, 0.14, 0.10
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