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Chipmunk Physics • View topic - Collision callbacks slow

Collision callbacks slow

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Collision callbacks slow

Postby J . » Tue Jul 20, 2010 7:53 am

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Re: Collision callbacks slow

Postby slembcke » Tue Jul 20, 2010 8:41 am

Well, callback are slow in that they tend to happen very often. 100 objects sitting in a pile can generate several hundred callbacks every frame. I'm sure viblio could give you a better answer about how Pymunk forwards the callbacks on to the interpreter, but there is almost certainly going to be plenty of overhead calling back and forth across the interpreter/C boundary. You should be wary of the Python profiler (and all profilers that work by wrapping individual calls) in that it tends to artificially inflate the runtime of small functions. The same thing can happen with Ruby's profiler, Unity's profiler, and gproff. Sometimes old fashioned benchmarks can give you more accurate profiling information.

Performance issues with callbacks is part of why I implemented the begin/separate collision messages. For people coding using bindings or people that are forwarding collision callbacks to expensive method calls (like Obj-C on the iPhone), having a simple callback that returns a static value can be almost as expensive or more than the collision detection itself. Receiving a single message on collision begin/separate was often all I wanted and putting the logic to track that into a Ruby callback was a very expensive way to track a boolean.

I've done a fair amount of Ruby game dev, and I wouldn't want to write a whole game in C. With Ruby at least, it's so slow that I've found that it's impractical to write a whole game in it. I often found that I had to rewrite a few functions in native code in order to get acceptable performance. If 5% of the code is using 50% of the CPU time, you don't have to do too much work to double the performance by rewriting it in a language that is 10x faster.
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Re: Collision callbacks slow

Postby J . » Tue Jul 20, 2010 2:36 pm

I wasn't focusing so much on that as on the fact that the profiler seems to indicate the overhead is coming from Chipmunk stuff running outside of the callback itself (that is, not called by it).

Edit: related, though not the reason I made the topic: after reading your post, I've just realised that part of the code in the pre-solve callback would be fine in a begin one, despite having considered it before and for some reason deciding to leave it alone. The rest only does stuff every fourth step of the space ATM, so it's not too bad. (I'm sure I'll find out why at a later date, after a few hours of debugging...)

While it would be nice to put some rewrite some intensive stuff in C, I think it's all quite spread out over a few functions, and a lot of the stuff taking the longest interacts with instances of Python classes, I've written. I haven't done bindings between languages before, but something tells me that might make it tricky. I'll look into it, though; performance hasn't been a priority up to now, but it makes sense to look into things before I end up at the point where it'll take a lot of refactoring to get good performance.
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Re: Collision callbacks slow

Postby viblo » Wed Jul 21, 2010 6:48 pm

http://www.pymunk.org - A python library built on top of Chipmunk to let you easily get cool 2d physics in your python game/app
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Re: Collision callbacks slow

Postby J . » Thu Jul 22, 2010 3:47 am

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Re: Collision callbacks slow

Postby viblo » Thu Jul 22, 2010 8:41 am

Ah, I think I understand it now, your question is why cf have so much cumtime when pre_collision doesnt, and why for example _get_shapes have more cumtime than pre_collision even when its only called from pre_collision?

I don't know, and I cant get it to happen with the simple test code I made.. Have you tried to profile with the profile module instead of cProfile?
http://www.pymunk.org - A python library built on top of Chipmunk to let you easily get cool 2d physics in your python game/app
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Re: Collision callbacks slow

Postby J . » Thu Jul 22, 2010 1:00 pm

Hmm, there is still some difference in the wrong direction, but not as much as with cProfile. I guess if different profilers can have such a discrepancy, the whole thing could be one?
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Re: Collision callbacks slow

Postby viblo » Thu Jul 22, 2010 6:05 pm

Maybe.. Guess the best would be to construct a small example and ask on the python list. Or just let it go ;)
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Re: Collision callbacks slow

Postby J . » Fri Jul 23, 2010 2:55 pm

Mhm...

Thanks for the replies, both of you - and the great software.
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