Parallelism

Parallelism simply means having two tasks run at the same time, or in parallel. The phrase "in parallel" is the key element that differentiates parallelism from concurrency. Why is this? Because parallel implies that two things are happening simultaneously. This is not the case in a concurrent system; in concurrent systems, you need to pause one task in order to let another continue execution. Note that this definition can be too simple and incomplete regarding the modern concurrent systems, but it is sufficient for us to give you a basic idea.

We meet parallelism regularly in our daily lives. When you and your friend are doing two separate tasks simultaneously, those tasks are being done in parallel. To have a number of tasks in parallel, we need separate and isolated processing units, each of which is assigned to a certain task. For instance, in a computer system, each CPU core is a processor unit that can handle one task at a time.

For a minute, look...