Parallelism Versus Concurrency

Lest our discussion of concurrency and parallelism lead you to think they are the same thing, let’s disentangle the two notions.

Concurrency is the coordination of multiple, usually interleaved threads of execution that are accessing or modifying some shared state.

Parallelism involves state as well, but usually in the inverse. Being an optimization technique used to efficiently utilize all of the available resources (usually computational, but sometimes other resources, like bandwidth) to improve the performance of an operation, approaches to parallelization generally aim to maximize the window of exclusive access to state (or, often, chunks of state) so as to minimize coordination overhead. Rather than involving interleaved threads of execution, the multiple evaluations of a parallelized operation run simultaneously—sometimes on different CPU cores, other times on different physical machines entirely.