Concurrency vs. Parallelism
The terms concurrency and parallelism are often used in relation to multithreaded programs. At first it may seem as if concurrency and parallelism may be referring to the same concepts. However, concurrency and parallelism actually have different meanings. In this concurrency vs. parallelism tutorial I will explain what these concepts mean.
Just to be clear, in this text I look at concurrency and parallelism within a single application - a single process. Not among multiple applications, processes or computers.
Concurrency means that an application is making progress on more than one task - at the same time or at least seemingly at the same time (concurrently).
If the computer only has one CPU the application may not make progress on more than one task at exactly the same time, but more than one task is in progress at a time inside the application. To make progress on more than one task concurrently the CPU switches between the different tasks during execution. This is illustrated in the diagram below:
Parallel execution is when a computer has more than one CPU or CPU core, and makes progress on more than one task simultaneously. However, parallel execution is not referring to the same phenomenon as parallelism. I will get back to parallelism later. Parallel execution is illustrated below:
