The terms concurrency and parallelism are often used in relation to multithreaded programs.Concurrency and parallelism are two related but distinct concepts.
Concurrency means, essentially, that task A and task B both need to happen independently of each other, and A starts running, and then B starts before A is finished.
There are various different ways of accomplishing concurrency. One of them is parallelism–having multiple CPUs working on the different tasks at the same time. But that’s not the only way. Another is by task switching, which works like this: Task A works up to a certain point, then the CPU working on it stops and switches over to task B, works on it for a while, and then switches back to task A. If the time slices are small enough, it may appear to the user that both things are being run in parallel, even though they’re actually being processed in serial by a multitasking CPU.
Concurrency means that an application is making progress on more than one task at the same time (concurrently). Well, 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 being processed at a time inside the application. It does not completely finish one task before it begins the next.
Parallelism means that an application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time.
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