[Vm-dev] Multiprocessing with Squeak

Levente Uzonyi leves at elte.hu
Fri Jan 29 13:17:46 UTC 2010


On Thu, 28 Jan 2010, Colin Putney wrote:

>
>
> On 2010-01-27, at 2:04 PM, Levente Uzonyi wrote:
>
>> Hi,
>>
>> I had an idea a few days ago and even though I don't have the time or knowledge to try it myself, I just can't get it out of my head. The idea is to let an interpreter use two images at once. One of them is read only "fully working" image let's call it S (source), the other is empty (contains no objects), writeable, possibly generated on the fly, let's call it W (working). The vm knows if an object is in S or W by checking the object pointer. Whenever an object in S is about to be modified, a copy is created in W and all references to it are changed to the new one (which means that more than one object might have to be copied). This means a slower startup, but once all necessary objects are copied performance would be normal.
>> (This approach is similar to the way sources are handled today: the sources file is read only, new source code goes to the changes file.)
>
> I believe VW can do something like this - they call it "Shared Perm Space." There's a special section of memory that's immutable, not subject to garbage collection, and shared between several VM processes.
>

After a bit of googling I found this:
http://cincomsmalltalk.com/userblogs/runarj/blogView?showComments=true&printTitle=Parallel_Execution_using_Multiple_VisualWorks_Images&entry=3348279474
and it looks similar, though it doesn't describe what Shared Perm Space is.

>> - combine it with HydraVM, it might give Erlang-like capabilities (cheap
>>  and fast processes)
>
> Well, we already have cheap and fast processes. The overhead for creating a new instance of Process and scheduling it is very low. What we lack is isolation between them. Squeak seems to be drifting in that direction, though. Islands are a good start. Josh's recent contribution of futures to the trunk are another step away from shared state concurrency.

Our cheap processes can't do multiprocessing and futures won't help with 
that.

>
> My sense of it is that efficient use of memory isn't the most important problem to solve at the moment. Further steps toward event-loop concurrency would be more fruitful.
>

I always see people saying: the image is large, we need a kernel image, 
squeak is bloated, I can't run this large image on a server, etc.

If a vm could do this, running 1000 images on a single server wouldn't 
hurt much (assuming ~15MB source image and <1MB worker images).


Levente

> Colin


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