[Stackless] Re: [Python-Dev] Stackless Design Q. (original) (raw)

Christian Tismer tismer@tismer.com
Fri, 22 Feb 2002 09:15:19 +0100

Greg Ewing wrote:

Christian Tismer <tismer@tismer.com>:

...

I can see the attraction of having pre-emption built in this way -- it would indeed be extremely efficient.

But I think you need to make a decision about whether your tasklet model is going to be fundamentally pre-emptive or fundamentally non-pre-emptive, because, as I said before, the notion of switching to a specific tasklet is incompatible with pre-emptive scheduling.

Yes. I will go a bit further and adapt the process model of the Alef language a bit.

If you want to go with a fundamentally pre-emptive model, I would suggest the following primitives:

[blocking stuff - ok]

Using this model, a coroutine switch would be implemented using something like

def transfer(t): "Transfer from the currently running tasklet to t." t.unblock() currenttasklet().block() although some locking may be needed in there somewhere. Have to think about that some more. For sending values from one tasklet to another, I think I'd use an intermediate object to mediate the transfer, something like a channel in Occam: c = channel() # tasklet 1 does: c.send(value) # tasklet 2 does: value = c.receive() Tasklet 1 blocks at the send() until tasklet 2 reaches the receive(), or vice versa if tasklet 2 reaches the receive() first. When they're both ready, the value is transferred and both tasklets are unblocked. The advantage of this is that it's more symmetrical. Instead of one tasklet having to know about the other, they don't know about each other but they both know about the intermediate object.

Yes. This all sounds very familiar to me. In private conversation with Russ Cox, Bell Labs, I learned about rendevouz techniques which are quite similar.

Having read the Alef user guide which can be found at http://plan9.bell-labs.com/who/rsc/thread.html http://plan9.bell-labs.com/who/rsc/ug.pdf I got the following picture:

(Thanks to Russ Cox, these are his ideas!) We use a two-level structure. Toplevel is something similar to threads, processes in Alef language. These are pre-emptively scheduled by an internal scheduler that switches after every n opcodes. These threads are groups of tasklets, which have collaborative scheduling between them.

This gives us a lot of flexibility: If people prefer thread-like behavior, they can use the system provided approach and just use the toplevel layer with just one tasklet in it. Creating new tasklets inside a process then has coroutine-like behavior. I'm just busy designing the necessary structures, things should not get too complicated on the C level.

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.

As I said before, if there are no references left to a tasklet, there's no way it can ever be switched to again, so its C stack is no longer relevant. Unless you can have return addresses from one C stack pointing into another, or something... can you?

Well, the problem is that an extension might be sitting inside a tasklet's stack with a couple of allotted objects. I would assume that the extension frees these objects when I send an exception to the tasklet. But without that, I cannot be sure if all resources are freed.

thanks a lot for your help - chris

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