What is the difference between concurrency and parallelism?

Question

What is the difference between concurrency and parallelism?

Examples are appreciated.

Answer 1

The simplest and most elegant way of understanding the two in my opinion is this. Concurrency allows interleaving of execution and so can give the illusion of parallelism. This means that a concurrent system can run your Youtube video alongside you writing up a document in Word, for example. The underlying OS, being a concurrent system, enables those tasks to interleave their execution. Because computers execute instructions so quickly, this gives the appearance of doing two things at once.

Parallelism is when such things really are in parallel. In the example above, you might find the video processing code is being executed on a single core, and the Word application is running on another. Note that this means that a concurrent program can also be in parallel! Structuring your application with threads and processes enables your program to exploit the underlying hardware and potentially be done in parallel.

Why not have everything be parallel then? One reason is because concurrency is a way of structuring programs and is a design decision to facilitate separation of concerns, whereas parallelism is often used in the name of performance. Another is that some things fundamentally cannot fully be done in parallel. An example of this would be adding two things to the back of a queue - you cannot insert both at the same time. Something must go first and the other behind it, or else you mess up the queue. Although we can interleave such execution (and so we get a concurrent queue), you cannot have it parallel.

Hope this helps!