Aha.  Ok.  Thanks everyone who answered.  This is what I was wondering.  I 
suppose looking back, it seems silly not to realize that since all 
instructions are passing through the VM that the VM would watch what a 
process is doing and preempt them if necassary.  But all the VM's I have 
been exposed to were purely cooperative (for speed reasons I suppose), so it 
didn't occur to me that it wouldn't be the same here.

>From: "Martin v. Löwis" <martin at v.loewis.de>
>Reply-To: The general-purpose Squeak developers 
>list<squeak-dev at lists.squeakfoundation.org>
>To: The general-purpose Squeak developers 
>list<squeak-dev at lists.squeakfoundation.org>
>Subject: Re: Preemptive scheduling
>Date: Tue, 01 May 2007 22:56:38 +0200
>
> > Does the Squeak VM do preemptive scheduling or regular yield style
> > scheduling.  This may sound like a silly question, but I know Erlang is
> > language that does mainly green threads and they are preemptive in
> > nature.  They accomplish this by allowing a given thread to execute no
> > more then 100 VM instructions before it is preempted (obviously if a
> > thread makes a primitive call that is a voluntary yield).
> >
> > Does Squeak do this already?
>
>Yes, the Squeak scheduler implements preemptive scheduling. An active
>process is given a quantum of such-and-such many byte codes, and when
>the quantum expires, a scheduling decision is made. The scheduler
>attempts to make each quantum a (tunable) period of time (by default
>1ms), and adjusts the number of byte code operations to account for
>differences in CPU speed.
>
>Regards,
>Martin
>

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