I have usually seen the distinction between concurrency and parallelism being drawn for precisely the opposite reason: parallelism without concurrency is a rela

I have usually seen the distinction between concurrency and parallelism being drawn for precisely the opposite reason: parallelism without concurrency is a relatively commonly used niche, it is much simpler than concurrency, and it has special tools that make sense just for it. For example, CUDA exposes a parallel programming model with low support for actual concurrency. Similarly, OpenMP is mostly useful for parallel computations that rarely need coordination. C#'s Parallel package is another example. By contrast, concurrency often rears its ugly head even in single-threaded workflows, even ones that are part of larger multi-threaded programs.