What is the difference between concurrency and parallelism?

Question

What is the difference between concurrency and parallelism?

Examples are appreciated.

Answer 1

In electronics serial and parallel represent a type of static topology, determining the actual behaviour of the circuit. When there is no concurrency, parallelism is deterministic.

In order to describe dynamic, time-related phenomena, we use the terms sequential and concurrent. For example, a certain outcome may be obtained via a certain sequence of tasks (eg. a recipe). When we are talking with someone, we are producing a sequence of words. However, in reality, many other processes occur in the same moment, and thus, concur to the actual result of a certain action. If a lot of people is talking at the same time, concurrent talks may interfere with our sequence, but the outcomes of this interference are not known in advance. Concurrency introduces indeterminacy.

The serial/parallel and sequential/concurrent characterization are orthogonal. An example of this is in digital communication. In a serial adapter, a digital message is temporally (i.e. sequentially) distributed along the same communication line (eg. one wire). In a parallel adapter, this is divided also on parallel communication lines (eg. many wires), and then reconstructed on the receiving end.

Let us image a game, with 9 children. If we dispose them as a chain, give a message at the first and receive it at the end, we would have a serial communication. More words compose the message, consisting in a sequence of communication unities.

I like ice-cream so much. > X > X > X > X > X > X > X > X > X > ....

This is a sequential process reproduced on a serial infrastructure.

Now, let us image to divide the children in groups of 3. We divide the phrase in three parts, give the first to the child of the line at our left, the second to the center line's child, etc.

I like ice-cream so much. > I like    > X > X > X > .... > ....
                          > ice-cream > X > X > X > ....
                          > so much   > X > X > X > ....

This is a sequential process reproduced on a parallel infrastructure (still partially serialized although).

In both cases, supposing there is a perfect communication between the children, the result is determined in advance.

If there are other persons that talk to the first child at the same time as you, then we will have concurrent processes. We do no know which process will be considered by the infrastructure, so the final outcome is non-determined in advance.