[Stackless] Re: [Python-Dev] Stackless Design Q.

Christian Tismer tismer@tismer.com
Thu, 21 Feb 2002 14:48:07 +0100

Jeff Senn wrote:

> Greg Ewing <greg@cosc.canterbury.ac.nz> writes:
>>It's not clear exactly what you're after here. Are you
>>trying to define the lowest-level interface upon which
>>everything else will be built? If so, I think what you
>>have presented is FAR too complex.
>>It seems to me you need only two things:
> ...
>>   t = tasklet(f)
>>   t.transfer()
> (Sorry if I missed something -- I've been *way* busy lately and
> haven't been giving this much attention -- that said...)
> But (if I understand the current plan) we will need mechanisms
> internal to the Python interpreter to transfer values and maintain
> blocked/running state anyway; since when you generate a tasklet and
> run it:
>  t = tasklet(f)
>  t.transfer()
> That may cause many more tasklets to be generated, run, and destroyed
> that you don't ever see ...  recursions/function calls in f, and
> only-Christian-knows what else...  so the transfer value mechanism
> might as well be built in.

I think all these little things are cheap to implement.

> I haven't thought enough about the "unamed produce-and-continue
> function" to decide how exactly it should work.

Somebody named it "resume", and together with "suspend" we get
a nice couple.
On the other hand: I'm not sure whether resume should block
its caller. I'm very undecided after all the input I got,
if it is in fact better to forget data transfer completely
by now and just make switching primitives which always work.

> I have two concerns in implementing uthreads this way (scheduler in
> C):
>  1 -- there doesn't seem to be anyway to "kill" a tasklet

Not yet, but I want to provide an exception to kill tasklets.
Also it will be prossible to just pick it off and drop it,
but I'm a little concerned about the C stack inside. This
might be the last resort if the exception doesn't work.

>  2 -- the scheduling algorithm will be hard to tune (we'll probably
>       *at least* need tasklet priority...)  Maybe there should still
>       be a "timeslice" function so an in-Python scheduler can be written?

We had the timeslice function, yes. I think to make things
simpler this time and just periodically call the scheduler
which is written in C. I also have a rough concept of
priorities which can be very cheaply implemented.
Maybe I implement some default behavior, but allow these
objects to be subclassed from Python?

ciao - chris

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