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Comment by pejrich | Hacker News Reader

The computer science definition of concurrency and parellelism are different that what you're explaining. In CS, a program can only be concurrent. Parallel is to do with how it's run, and the hardware. A concurrent program, is still concurrent even if it's run on a single core, whereas to be considered parallel, it must be running on multiple cores/processors/machines/etc. at the same time.

One way to think of it: If I have three plates of food, that I can eat in any order, but it's just me eating, then this is concurrent. In practice, this is no faster than if I had to eat 1, then 2, then 3, however it has the potential to be. By adding another person, it becomes parallel.

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When I was in computer science classes in college, "concurrency" was defined as multiple processes running at the same time on multiple processors. You could not have concurrency with a single core. If all you had was a single core, then the best you could hope for was "pseudo-concurrency," because it is impossible to run two things simultaneously on one core. When did this definition change?

Your food metaphor doesn't really work in the computer science sense, because you cannot concurrently eat three things at the same time unless you put them all on the fork at the same time, which is something you simply cannot do in computer science.

It never did. A concurrent program has always meant a decomposition of a program into parts that can be executed out-of-order while maintaining the same output. Whether or not this is by some scheduler on a single-core cpu is irrelevant to the theory.

The words "concurrency", "multi-threaded", and "parallel" each have different meanings. You can have a program that is multi-threaded but which does not maintain concurrency ie. the output is dependent on race conditions. This is usually, but not always, a bug.

Your confusion stems from the fact that most people do not care about the distinction. You usually don't care about concurrency unless you plan to actually run things in a manner which is unordered. Thus when the theory says "concurrency" you think "parallel". Technically speaking you're wrong, practically speaking your mistake usually won't matter.

As a final example consider a single-core computer running Windows with multiple processes running a myriad of services and programs. At any one time only a single process can run on that single available core, but in practice they are running "at the same time", because the system is implementing a model of concurrency allowing it to schedule and execute the different processes out-of-order.

You don't understand the food metaphor. If you have 3 plates of food, but they can be eaten in any order, then 3 people can eat them in 1/3 the time that 1 person could eat them. The food/plate is the code/data, the person/fork is the processor. You're never eating the same exact piece of food at the same time.

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