Concurrency vs Parallelism

An excerpt from Grokking Concurrency by Kirill Bobrov

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The conversational meanings of the words “parallel” and “concurrent” are mostly synonymous, which is a source of significant confusion that extends even to the computer science literature. Distinguishing between parallel and concurrent programming is very important because both pursue different goals at different conceptual levels.

Concurrency

Concurrency is about multiple tasks which start, run, and complete in overlapping time periods, in no specific order. Parallelism is about multiple tasks or subtasks of the same task that literally run at the same time on a hardware with multiple computing resources like multi-core processor. As you can see, concurrency and parallelism are similar but not identical.

Concurrency is a semantic property of a program or system. Concurrency is when multiple tasks are in progress for overlapping periods of time. Note, here we are not talking about the actual execution of the tasks, but the design of the system — that the tasks are order-independent. So, concurrency is a conceptual property of a program or a system, it’s more about how the program or system has been designed.