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3.1. VM Support for Concurrent and Parallel Programming
considered internal basic building blocks outside of the scope of this survey,
and thus do not have an influence on our results.
3.1.3. Conclusion
Summary This survey investigated the state of the art in VM support for
concurrent and parallel programming. It examined thirteen VMs, including
contemporary multi-language VMs and a number of VMs selected 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 the standard libraries. The main insight is
that the analyzed VMs only support one or two categories of concepts. Furthermore,
they consistently relegate support for parallel programming to the
standard library without providing explicit support for optimization.
Concept Exposure In answering the question of how concepts are exposed,
the survey shows that very general concepts such as shared memory, memory
models, and global interpreter lock semantics are realized by a combination
of mechanisms in the VM, which were categorized as implicit semantics
(cf. Sec. 3.1.1.1). Typically, they have an impact on most parts of a VM, because
they require guarantees from a wide range of VM subsystems. More
restricted, and often performance sensitive concepts are exposed as part of
the overall instruction set architecture. Examples are monitors, synchronized
methods, volatile variables, and in some cases also high-level concepts like
channels, message sends, message receives, and threads or processes.
Primitives are used for a wide range of concepts. Design decisions differ
between VMs, thus some concepts are supported either in the instruction
set or as primitives, e. g., locks, channels, and threads, but also concepts like
atomic operations, and condition variables. Other high-level concurrency concepts
such as concurrent data structures are provided as libraries. With the
definitions of concurrent and parallel programming of Sec. 2.3 in mind, the
conclusion is that none of the concepts that are provided with implicit semantics,
ISA support, or primitives is directly related to parallel programming.
Instead, all the identified parallel programming concepts have been realized
in the form of libraries.
Only limited support for concurrent and parallel programming. Since support
for parallel programming is based on libraries, none of the VMs is cate-
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