[Python-Dev] Re: [Stackless] comments on PEP 219

[Christian Tismer]

Guido van Rossum wrote:
> 
> > Christian Tismer <tismer@tismer.com>:
> >
> > > But stopping the interpreter is a perfect unwind, and we
> > > can start again from anywhere.
> >
> > Hmmm... Let me see if I have this correct.
> >
> > You can switch from uthread A to uthread B as long
> > as the current depth of interpreter nesting is the
> > same as it was when B was last suspended. It doesn't
> > matter if the interpreter has returned and then
> > been called again, as long as it's at the same
> > level of nesting on the C stack.
> >
> > Is that right? Is that the only restriction?
> 
> I doubt it.  To me (without a lot of context, but knowing ceval.c :-)
> it would make more sense if the requirement was that there were no C
> stack frames involved in B -- only Python frames.

Right. And that is only a dynamic restriction. It does not
matter how and where frames were created, it is just impossible
to jump at a frame that is held by an interpreter on the C stack.
The key to circumvent this (and the advantage of uthreads) is
to not enforce a jump from a nested interpreter, but to initiate
that it will happen. That is, the scheduling interpreter
does the switch, not the nested one.

ciao - chris

-- 
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