Concurrent tasks complete in the same, overlapping time period.
Parallel tasks run at literally the same time.
I don't really agree. The heart of the problem of concurrency is non-determinism, but it's perfectly possible to have deterministic parallel algorithms. Normally the key is not letting the parallel operations interact with one another.
So to me, a useful (for discussion) definition of concurrency involves multiple logical tasks, overlapping in time, and interacting with one another, in a non-deterministic way.
Whereas parallelism is concerned with taking advantage of physical hardware that can do more than one thing simultaneously.
And concurrency does not imply parallelism, nor does parallelism imply concurrency, under my understanding. In particular, data parallelism like SIMD or CUDA is not concurrent.
Hmm, I would think it would be the opposite, they're concurrent precisely because they don't have to interact. They can run independently. 2 requests from a server are concurrent because they don't have to know about each other and don't have to interact with each. This is a property of the problem domain (idealized web requests) this doesn't tell us anything about how they'll run (in parallel or not).
> Whereas parallelism is concerned with taking advantage of physical hardware that can do more than one thing simultaneously.
I agree with that.
> And concurrency does not imply parallelism, nor does parallelism imply concurrency, under my understanding. In particular, data parallelism like SIMD or CUDA is not concurrent.
Don't quite agree with that and don't see why SIMD algorithms have to be a special case. Maybe you compute a dot product between 2 vectors. If you write the algorithm down you have a bunch of multiplications and a sum. You notice that it has a lot of concurrency (the algorithm). If you don't have SIMD you could spawn a thread to multiply out each pair and then to sum. That would be silly. But you'd run in parallel. You could just do it sequentially with a for loop. But if you have SIMD, it know how to run those concurrent algorithmic steps in parallel.
