I'm sure there was mail earlier asking about when a process switch  
happens. Ok let's clarify that. There is only one place in the VM  
where that happens is.

Interpreter>>transferTo:

called by

primitiveSuspend		->  transfer to wakeHighestPriority
	Called via Smalltalk Process>>suspend, Process>>terminate

primitiveWait			->  transfer to wakeHighestPriority, unless  
excessSignals is > 0, in that case we subtract one, and don't do the  
transferTo:
primitiveYield		->  transfer to wakeHighestPriority, and stick  
current process on last link of queue in it's priority group.
resume:				-> transer to given process if it has higher priority than  
current process, and put current process toSleep:.
					Otherwise if same or less prioirty put given process toSleep:

	Called on a Semaphore signal via Interpreter>>synchronousSignal:  
This ensures if the process is higher priority it will become un- 
suspend and run, or
	become un-suspended and be put on queue to run some day later...

I'll note the priority queue is a sorted list of linked lists.

Helper methods

toSleep:  	-> sticks process on last link of priority queue.

wakeHighestPriority  	-> fetchs the process that is runable that is  
the highest priority. Ending of course with the lowest priority idle  
loop process which is always runable. Failure to fine any runnable  
process terminates the VM with an error.



Semaphores

There are semaphores that are signaled via checkForInterrupts which  
we mentioned earlier that attempts to run every 1ms.

special semaphores are:
lowSpace   Signal if space is low
interruptPending  Signal if keyboard interrupt was done
TheFinalizationSemaphore,   Signal if any GC Finalization needs to run.

The other two interesting semaphores are:

ExternalSemaphores, 	dual queue of semaphores set usually by plugins  
that want to signal a semaphore.  SocketPlugin is heavy user of this  
logic.  The reason for two queues
						is one queue is active, the other is being processed, this  
avoids (one hopes) a race condition between signalling and processing  
because the VM has no
	 					way to lock access to the queue with a thread safe access  
method since that level of processor instruction support  is not in  
the basic VM.

TheTimerSemaphore,   The Delay logic tracks when the next Delay will  
fire, it feeds that information to the VM, checkForInterrupts by  
waking up every 1ms then looks at the
request time, versus ms time, and decides if it needs to signal the  
TheTimerSemaphore, which then  waits up the process waiting on the  
Delay.



CheckForInterrupts is run when

a) A method starts to run
b) When we do a long unconditional jump bytecode (#160-167) backwards.

However that is moderated by interruptCheckCounter which tries to  
ensure not every call will result in a checkForInterrupts call.
As earlier discussed over the years  
interruptCheckCounterFeedBackReset tries to moderate the setting of  
interruptCheckCounter
to ensure for example a do:while:  which would result in two (if not  
more) checkForInterrupt calls on each loop, the fiddling with  
interruptCheckCounter will try to
ensure checkForInterrupt is only called every 1ms.

We use the forceInterruptCheck to force a checkForInterrupt call,  
that is done on finalization signaling, lowspace, become:,  
incremental/full GC, exernal semaphore signalling, plus a few others.

On May 29, 2007, at 11:28 AM, J J wrote:

> Hi all,
>
> I just read http://wiki.squeak.org/squeak/5682 that Tim (I think it  
> was) mentioned, and found the part about doing alternative  
> scheduling with a higher priority thread interesting.
>
> The question I had was, in the example it says the high priority  
> process can use Delay's to sleep while other processes are running,  
> but what happens when a process does IO?  Doesn't that cause the  
> process to yield?
>
> What I am thinking is, if I were to make a scheduler process that  
> manages all running processes, how can he find out every situation  
> where a process yields in some way?  It would be ideal if there was  
> a Delay millisecondsOrUntilInterupt: type message.
>
> Thanks,
> Jason
>
> _________________________________________________________________
> More photos, more messages, more storage—get 2GB with Windows Live  
> Hotmail. http://imagine-windowslive.com/hotmail/?locale=en- 
> us&ocid=TXT_TAGHM_migration_HM_mini_2G_0507
>
>

--
======================================================================== 
===
John M. McIntosh <johnmci at smalltalkconsulting.com>
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
======================================================================== 
===