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3.2. A Survey of Parallel and Concurrent Programming Concepts
of each other and require significantly different research, this dissertation
chooses to focus on guaranteeing the semantics, i. e., the correctness of implementations
for concurrent programming concepts on top of multi-language
VMs.
3.2.1. Survey Design
The goal of this survey is to identify concepts that are relevant for a multilanguage
VM in order to facilitate the implementation of concurrent and
parallel programming abstractions. To that end, this section first discusses
the selected questions that categorize the concepts. Then, it details the approach
to identify parallel and concurrent programming concepts and finally,
it presents the findings and concludes with general requirements for the support
of these concepts.
3.2.1.1. Survey Questions
When features are considered for inclusion in a VM, one of the main goals is
to avoid unnecessary complexity (cf. Sec. 2.1). From this it follows that a new
concurrent or parallel programming concept needs to be added to a VM only
if it cannot reasonably be implemented in terms of a library on top of the VM.
Thus, our first question is:
Lib Can this concept be implemented in terms of a library?
Interpreting the question very broadly, it considers whether some variation
of the concept can be implemented, i. e., a variation for instance with missing
semantic guarantees such as isolation. Thus, such a library implementation
can either suffer from losing semantic guarantees, or it has to accept performance
drawbacks. The following two questions account for this variation:
Sem Does this concept require runtime support to guarantee its semantics?
Perf Are there indications that runtime support would result in significant
performance improvements compared to a pure library solution?
The answers to Sem also consider interactions of different languages on top
of a VM as well as the influence of reflection. This is relevant since language
guarantees are often enforced by a compiler and do not carry over to the
level of the VM. One example is the semantics of single-assignment variables,
which is typically not transferred to the bytecode level of a VM.
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