[Stackless] tasklets / greenlets vs. threads / microthreads

Christian Tismer tismer at stackless.com
Tue Mar 16 15:37:12 CET 2004

Andy Sy wrote:
> Can someone give a short summary of how tasklets are different from
> microthreads (i.e. what are their pros and cons)?  From what I
> remember with playing around with Will Ware's uthreads module a couple
> of years ago, microthreads offer a very similar interface and
> functionality as normal threads (as the name would imply).
> How do tasklets and greenlets compare to threads/microthreads?  Can they
> be used for the same things?

Microthreads try to behave like threads. This includes
pre-emptive scheduling.

In Stackless 2.0, tasklets could not do pre-emptive scheduling,
because I had no good criterion when to wllow and forbid it.
In Stackless 3.0 it is possible again, since the non-recursive
interpreter calls give me some measure of an "innocent" state.
Richard Emslie implemented pre-emptive scheduling in the
sprint, and it is of course possible (and a planned task)
to implement microthreads on top of that.

This doesn't mean that I like threads at all, it is just that
people are asking for it. I personally prefer explicit task
switching over implicit, but there are a few applications
which really make sense, for instance, a background task
that automagically saves pickles of a running program,
and yo don't need to change the program for this service.

Tasklets are meant in a way like a simplified subset of
threads, and by design they are chained together into
a circular list for round-robin scheduling.
On the sprint it came up that this is even too much,
and we should have a simpler interface, where tasklets
can be built upon. We had an indea of "corolets", small
coroutine like thingies which just can be switched passsing
a value.

Then the greenlets came up. They basically are these
corolets, but they come together with a new way of how to
hard-switch the C stack, and they generalize the problem
of how to soft-switch at the same time.

Yes, greenlets will be the building block for tasklets
and microthreads.

ciao - chris

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