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2.3. Concurrent vs. Parallel Programming: Definitions
were mostly concerned with small-scale systems that used time shared execution.
Following their argumentation, the term parallelism on the other hand
comes from the supercomputing community, which was mostly concerned
with large-scale systems and physically parallel execution of programs. However,
for the purpose of their book, they conclude the discussion by saying:
“we will use the terms [concurrent and parallel] interchangeably to refer to logical
concurrency”.
Definitions of Sottile et al. Overall, the field distinguishes the two terms
based on the underlying execution model. A set of concrete definitions is
given by Sottile et al. [2010, p. 23-24]. They write: “We define a concurrent
program as one in which multiple streams of instructions are active at the same
time.” Later, they go on and write: “Our definition of a parallel program is an
instance of a concurrent program that executes in the presence of multiple hardware
units that will guarantee that two or more instruction streams will make progress
in a single unit of time.” While these definitions are seldom spelled out precisely
in literature, the definitions of Sottile et al. seem to reflect the common
consensus of what concurrency and parallelism mean. Therefore, parallel programs
are considered to be a subset of concurrent programs. For instance,
Sun Microsystems, Inc. [2008] uses very similar definitions.
Not all parallel programs are concurrent programs. The major issue of
characterizing parallel programs as a subset of concurrent programs is the
implication that all parallel programs are concurrent programs as well. Thus,
all parallel programs can be mapped on a sequential execution. However,
there are useful algorithms such as the elimination stack proposed by Shavit
and Touitou [1995b] that are not strictly linearizable [Herlihy and Wing, 1990].
Which means, parallel programs can have executions that cannot be replicated
by any concurrent program, i. e., they cannot be mapped on a sequential execution
without losing semantics.
Current definitions do not add value to explain programming concepts. In
addition to the conceptual problem of characterizing parallel programs as a
subset of concurrent programs, the notion of a subset relation does not add
explanatory value in practice, either.
For example, Cilk’s fork/join with a work-stealing scheduler implementation
[Blumofe et al., 1995] is designed for parallel programming. The idea of
fork/join is to recursively divide a task to enable parallel execution of sub-
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