[Python-ideas] Python and Concurrency

[Josiah Carlson]

"Lucio Torre" <lucio.torre at gmail.com> wrote:
> > I'm sure that there's a lot more like that out there. However, there is
> > also a lot of stuff out there that is *superficially* similar to what I
> > am talking about, and I want to make the distinction clear. For example,
> > any discussion of concurrency in Python will naturally raise the topic
> > of both IPython and Stackless. However, IPython (from what I understand)
> > is really about distributed computing and not so much about fine-grained
> > concurrency; And Stackless (from what I understand) is really about
> > coroutines or continuations, which is a different kind of concurrency.
> > Unless I am mistaken (and I very well could be) neither of these are
> > immediately applicable to the problem of authoring Python programs for
> > multi-core CPUs, but I think that both of them contain valuable ideas
> > that are worth discussing.
> 
> From what i understand, i think that the main contribution of the
> stackless aproach to concurrency is microthreads: The ability to have
> lots and lots of cheap threads. If you want to program for some huge
> amount of cores, you will have to have even more threads than cores
> you have today.

But it's not about threads, it is about concurrent execution of code
(which threads in Python do not allow).  The only way to allow this is
to basically attach a re-entrant lock on every single Python object
(depending on the platform, perhaps 12 bytes minimum for count, process,
thread).  The sheer volume of the number of acquire/release cycles
during execution is staggering (think about the number of incref/decref
operations), and the increase in size of every object by around 12 bytes
is not terribly appealing.

On the upside, this is possible (change the PyObject_HEAD macro,
PyINCREF, PyDECREF, remove the GIL), but the amount of work necessary to
actually make it happen is huge, and it would likely result in negative
performance until sheer concurrency wins out over the acquire/release
overhead.


 - Josiah