Contextualizing the VM environment?

[Colin Putney]

On Apr 13, 2006, at 4:53 PM, Ian Piumarta wrote:

> Hi Andreas,
>
>> What would happen if we changed the VM so that we fetch the  
>> specialObjects array from the active process? In other words, each  
>> process could potentially carry its own VM environment which  
>> effectively describes its own variant of the VM state. With the  
>> effect being that different processes could potentially have  
>> different versions of class SmallInteger etc.
>>
>> The basic question here is: Can you guys see any reason why this  
>> *wouldn't* work? Is there anything I'm overlooking? Any situations  
>> in which this would cause problems while (say) interrupt  
>> processing or somesuch?
>
> The spirit of this I like a lot.
>
> A few things off the top of my head...  The VM caches some objects  
> for speed (nil, true, false) that you'd have to reload at process  
> switch.  You still wouldn't have complete isolation of the VM  
> internals between processes.  They'd all have to agree on the  
> processor scheduler (or keep activeProcess apart from it); the  
> formats of the splObj classes couldn't change too much (changes in  
> splObjs do not affect the assumptions in the way primitive access  
> fixed fields -- or you could make the primitive table process-local  
> too ;-); compact classes would have to be the same (or non- 
> overlapping) for any pair of processes that share instances;  
> semaphores would have to be reloaded at process switch unless you  
> admit the possibility of some processes hijacking interrupts, low  
> space, etc. from the installed handlers.
>
> Of course, the correct solution is to get rid of the VM entirely.   
> (By assuming the splObjs subsume the interesting aspects of VM  
> internals and then making them process-local, you are essentially  
> trying to pull most of the VM implementation up into userland with  
> per-process granularity on customisation of its behaviour.)

Hi Andreas,

I have a question along the lines of Ian's observation above. How  
does this idea differ from what you proposed after Craig posted his  
1337 image a while back? (http://lists.squeakfoundation.org/pipermail/ 
squeak-dev/2006-January/099832.html) In both cases, you're talking  
about, essentially, a VM instance per Process. Is this another  
approach to the same idea, or is there a key difference that I'm  
missing?

Colin