here - Stefan-Marr.de

4. Experimentation Platform
Another reason for its choice are the experiments that were performed in
addition to the work on this dissertation. Examples are the implementation
of a pauseless garbage collector [Click et al., 2005] to avoid stop-the-world
garbage collection pauses, 5 as well as a work-stealing scheduler to improve
the performance of fine-grained parallelism. 6 Especially relevant for the work
presented here was an experiment to use operating system processes instead
of threads as the underlying abstraction to use multiple cores (cf. Sec. 4.4.3). 7
These extensions to the RoarVM promise to be a good foundation to experiment
with different optimizations (cf. Sec. 9.5.3).
4.4.1. Execution Model, Primitives, and Bytecodes
This section discusses technical details that are relevant for the implementation
of the OMOP and its performance evaluation.
Execution Stack Representation Similarly to the execution model of SOM,
the RoarVM uses the classic Smalltalk-80 model of context objects that represent
stack frames. Each frame, i. e., context object is a standard Smalltalk
object allocated on the heap and subject to garbage collection. They represent
the method activation, the operand stack, and temporary variables. A context
object also encapsulates the corresponding instruction and stack pointer. To
reduce the pressure on the garbage collector (GC), context objects are cached
and reused if possible, instead of leaving them for the GC.
While using objects to represent the execution state enables for instance
metaprogramming, it comes with a performance cost. To reduce that cost by
avoiding frequent indirections on the context objects, part of the execution
state such as instruction pointer and stack pointer are replicated in the interpreter
object and maintained there. The context object is only updated with
the execution state when necessary. Thus, the execution state is written to the
actual context object before garbage collection starts, before scheduling operations,
such as resuming a processes or yielding execution, might change the
currently active context, and before a message send activates the new context
for its execution.
Primitives and Quick Methods Primitives are used to implement functionality
that cannot be implemented directly in Smalltalk or for functionality
5 https://github.com/smarr/RoarVM/tree/features/parallel-garbage-collection
6 https://github.com/smarr/RoarVM/tree/features/scheduler-per-interpreter
7 https://github.com/smarr/RoarVM/tree/features/processes-on-x86
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