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3. Which Concepts for Concurrent and Parallel Progr. does a VM need to Support?
mean that in certain situations reflective state changes need to be tracked in
the same way that normal state changes are tracked.
Tab. 3.7 summarizes these requirements briefly.
3.5. Conclusions
VMs support only concurrent programming concepts directly. Parallel programming
concepts are provided in libraries only. Sec. 3.1 investigated the
state of the art in VM support for concurrent and parallel programming. The
survey examined thirteen VMs, including the contemporary multi-language
VMs, and a number of VMs that are reputed for their support for concurrent
and parallel programming. The analysis identified for each VM the concepts
it exposes and whether it exposes them in terms of implicit semantics, as part
of the VM’s instruction set architecture, in terms of primitives, or merely as
part of standard libraries. The major insight is that the surveyed VMs support
only one or two categories of concepts. Furthermore, they consistently
relegate support for parallel programming to their standard library without
providing explicit support for optimization.
Another observation is that common language guarantees such as isolation
for actors or the constraints around the STM system of Haskell are realized as
part of these VMs. Thus, the VM has full control over the exact set of provided
guarantees. however, providing support for all conceivable concepts is not
desirable for a multi-language VM. Problems with complexity and feature
interaction make it infeasible, as argued in Sec. 2.1.
Parallel programming concepts benefit from performance optimizations,
while concurrent programming concepts require semantic enforcement.
Sec. 3.2 discussed a wide range of concepts for concurrent and parallel programming.
Concepts are categorized by answering the question of whether
their semantics benefit from an enforcement in the VM and the question of
whether their performance could benefit significantly from VM support. The
result is that parallel programming concepts benefit most from performance
optimization, while concurrent programming concepts require support from
the VM to enforce their semantics.
The concepts that require support from the VM to guarantee their semantics
were chosen as the focus of this dissertation. Therefore, the remainder of this
dissertation investigates a unifying substrate for concurrent programming.
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