[Vm-dev] Efficient thread-local shared variables

Andreas Raab andreas.raab at gmx.de
Tue Oct 24 04:46:26 UTC 2006

Folks -

For a variety of reasons I am in dire need of the ability to vector 
shared variables (globals, class vars and pool vars) through an extra 
indirection vector per process (really per island but binding per 
process seems to be simpler for now). Since I need this for *each and 
every shared variable* it needs to be *very* efficient.

The question is: What is the most efficient way to implement such a 
scheme? There are a couple of ways I can think about:

1) Just use a dictionary. The main disadvantage is the lookup cost which 
could be handled by making it a special kind of dictionary and 
implementing the lookup in a primitive. This is a good fallback position 
but probably just a little slow in general. It could implemented by 
something along the lines of:

ProtoObject>>lookup: sharedBinding
     "Look up the value of the given shared binding in the currently 
executing process."
     ^Processor activeProcess scope at: sharedBinding ifAbsent:[nil].

which is pretty straightforward.

2) Use message lookup, e.g., send a message. This is simple to describe 
but not necessarily simple to implement correctly. Here is how the 
simulation would look like:

ProtoObject>>lookup: sharedBinding
     "Look up the value of the given shared binding in the currently 
executing process."
     ^[Processor activeProcess scope perform: sharedBinding key]
        on: MessageNotUnderstood do:[:ex| ex return: nil].

One problem here is that the key needs to be unique within all possible 
keys which is a problem if there is a name conflict. This can be 
resolved by implicitly prefixing names with the place where they are 
defined so it's not such big of a deal conceptually but practically the 
impact of that change might be more visible.

The other problem is that the scope object needs to hold all the objects 
which means quite a number of them. OTOH, one could argue that in many 
ways "Smalltalk" is just an object with a few thousand iVars so having a 
class representing the namespace defined by Smalltalk may be quite 

3) Use "some" integer index caching scheme. The main idea here is in 
realizing that really, option #2 doesn't quite work since classes can't 
have more than 256 iVars so we'd need to have an indirection through an 
array to be able to access these variables. If that is so, then why 
can't we inline the entire access pattern and have the scope just be an 
array that we index directly?

This is actually the most interesting approach to me because (as far as 
I can tell) it would be by far the most efficient. The basic idea goes 
like this: If all shared variables are assigned a "global index" then 
only this index is required to use them. Any use of the shared variable 
Foo would be inlined to "Processor activeProcess scope at: FooIndex" 
which (given proper primitive support) would probably be by far the 
fastest version (if offered a byte code it should rival the current 
speed of accessing shared variables). [I'll admit that there are some 
tricky issues with this approach as well, like the size needed for the 
scope object and whether or not to use hash lookup instead of indexing]

In any case, I'm trying to gather options. If any of you have any new 
ideas or have tried one or the other (successfully or not) or have any 
other comments to make I'd love to hear about it.

   - Andreas

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