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7. Implementation Approaches
introducing it was not performance, but the system’s debuggability needed
to be improved.
Since most domains customize only a subset of all intercession handlers, it
becomes a major problem to have all intercession handlers be invoked. While
the standard intercession handlers only encode the semantics of unenforced
Smalltalk execution, they significantly increase the amount of code that is
executed for each operation with a corresponding intercession handler. For
the purpose of debugging, the execution of this code only adds noise and
results in additional code execution that one needs to step through, without
relevance to the semantics.
During debugging at the VM bytecode level as well as during debugging
at the Smalltalk source level, it was essential to avoid such noise in order
to be able to focus on the problem at hand. The customization constant reduced
unnecessarily executed code significantly and consequently improved
the debugging experience for code executed in the enforced mode.
Representation of Ownership The RoarVM+OMOP represents object ownership
by using an extra header word for every object. While this direct approach
enables the VM to obtain the owner simply by object inspection, it
comes with a significant memory overhead for small objects. In the field of
garbage collection, different solutions have been proposed to track similar
metadata for various purposes. One possible approach that is common for
GCs is to partition the heap [Jones et al., 2011, chap. 8] based on certain criteria
to avoid the direct encoding of related properties. For the OMOP, the
ownership of an object could be encoded by it being located in a partition
that belongs to a specific domain. Such an approach would avoid the space
overhead of keeping track of ownership. While partitioning might add complexity
to the memory subsystem of the VM, it might also open up opportunities
for other optimizations. Depending on the use of the domains, and
the concurrency concepts expressed with them, a GC could take advantage of
additional partitioning. For instance, in an actor-like domain, additional partitioning
could have benefits for generational collectors. In such a system, the
nursery partition would be local to a single actor, which could have a positive
impact on GC times.
Limitations of Primitives A final semantic issue is the handling of primitives.
By reifying their execution and requiring the indirection to the domain
object, the execution context changes when the primitive is applied. In
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