[Sidefx-houdini-list] Python in Houdini 9

Pablo Giménez pablogipi at gmail.com
Thu Nov 2 06:23:07 EST 2006

Thanks Lutz, quite interesting.
I hope that somebody from SESI can give us any opinion about how to
implement Python into Houdini.

On 10/31/06, Lutz Paelike <lutz_p at gmx.net> wrote:
> > Have you tried stackless?
> Yes i know the creator of stackless python, Christian Tismer,
> very well and did a major development project with him (non 3d).
> > The performance is much better than regular python using threads?
> The performance can be better if used wisely, especially for simulations.
> There are some different paradigms involved with stackless.
> One core feature is the ability for unlimited recursion.
> Normally the execution context in Python (aka a frame object)
> is moved on the stack when you call another function and restored
> when you leave it. In Stackless Python however the internal implementation
> does not follow the regular scheme. Python stack frames are managed internally
> more like a linked list. That means context switching is *extremely* fast
> which makes it perfect for simulations and the overall performance is also better.
> By some magic hackery fast context switching also works with
> Python Modules written in C (when using the stackless C api it is less hackish)
> but you have less trouble if you stay in the python world.
> A problem arises if you have long lasting calls (especially into C-Extensions).
> You can play well with stackless python if you do only simple operations and return
> quickly, this applies as well to standard python generators (some sort of stream processing).
> Imagine you have a particle system with 100.000 particles.
> It would be nice to have a thread per particle to put the simulation logic
> in a class and have an array of "agents" and just cycle through all agents to evaluate.
> With regular threads this would be impossible to do since the memory consumption
> would kill your machine, but there is a solution in stackless called microthreads.
> Microthreads are no proper system level threads, but just lightweight pseudo-threads
> with a memory footprint of about 5kb (depends on your class implementation of course).
> That means you can keep easily 100.000 simple microthreads in 512 MB memory. (very nice)
> The scheduling of the microthreads is up to you. By implentation you have a cooperative scheduling model,
> but of course it is up to you to implement more advanced scheduling easily in python.
> (you could decide to not evaluate non-visible agents at all to speed up the simulation)
> Another killer feature is that microthreads can be pickled !
> (pickling is the python term for conveniently serializing (saving to disk) almost everything).
> That means you can stop a running simulation at any time and dump it to disk, load it back later
> and continue simulation. The computer which loads the pickled microthreads must not necessarily
> be the same where you saved it, since pickles objects are platform independent.
> Aside from just loading and saving to disk you could also choose to send a pickled microthread
> through the network to another machine and implement some kind of load-balancing.
> So you create some really nifty simulation farm !
> Don't forget you are still not bound to the python world but can implement
> heavy computing tasks in C extensions. As long as the extension is present on
> every machine in the pool it is all good.
> > It will be great if SESI cretes a solid core base for Houdini 9 ready
> > for concurrent programming, and transparent for developers.
> > For example, providing in the HDK functions that runs in parallel when
> > houdini cooks,or like in Maya where you can ensure that your
> > operations ar concurrent safe so Maya try to parallel these
> > operations.
> Exactly. If the feature list  for Houdini 9 is still not carved in stone
> then i would really encourage SESI to provide some API to make it possible
> to hook into the core houdini event loop or at least to register callbacks for specific events.
> I don't know the existing Houdini API very well yet, so maybe it is already there.
> Thereby it would be possible to integrate various custom libraries and gui kits (like wx and qt)
> or to integrate better.
> I am looking at this specific problem for some time now and can say it is a burden
> to integrate deeply into an application without having access to the core event loop of
> commercial software without access to the source.
> Trolltech provides support for Qt (only commercial version, not GPL)
> to hook up the Qt event loop into X based Applications but it would be *much* better
> to have official support  in the hosting application for this.
> *sigh*
> > Multiprocessing is the future and is almost a comodity feature in
> > computers today, so I think that is very important that Houdini 9 can
> > respond to these new times.
> Don't forget that there is still the Global Interpreter Lock (GIL) in Python.
> You *can* have multiple system level threads (or stackless microthreads)
> but as long as you evaluate python bytecode there can only be *one* active thread per process.
> That means execution of python code is serialized by the GIL.
> This means also that threads don't scale your python program as well as in
> other environments but make it somewhat simpler to handle.
> >From time to time people demand to get rid of the GIL but this will not gonna happen
> (at least  not in the current implementation of python, aka CPython)
> there are other implementations out there, which try to solve the problem differently
> but that's another story...
> A saner and more stable approach (for the host environment, houdini in this case)
> would be to run your custom simulation in a separate process
> on a another processor (core or computer) and bridge it with a network connection.
> Hope you got a brief overview of stackless python for the moment.
> By the way the Massive Multiplayer Online Game "Eve Online"
> runs on stackless python, on the the server side to keep a persistent simulation for
> the online gamers.
> The creator of EVE,  CCP Games is a major contributor to stackless.
> see 7.4 at http://www.eve-online.com/faq/faq_07.asp
> Cheers,
> Lutz
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Un saludo
Best Regards
Pablo Giménez

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