[adrusi]

Well obviously threads aren't always running at the same time since each cpu can do only one thing at a time (or a finite number, if we're counting hardware threads) and you can have more threads than cpus.

It's the fact that they might run at the same time. Also from the perspective of the programmer, there's no difference between two threads running at the same time or by time sharing, since preemptive multitasking is non-deterministic and has the same implications as true parallelism.

[jallmann]

I think we're talking about different things? You seem to be referring to parallelism in the literal, general sense of the word, while I'm referring to computational parallelism. Basically, if Amdahl's Law doesn't apply, then you're looking at concurrency. So this statement

> from the perspective of the programmer, there's no difference between two threads running at the same time or by time sharing

is incorrect when you're talking about computational parallelism (as OP was), because you're not going to realize any speedups with time sharing. In that case, you're using threads as a concurrency mechanism -- not for parallelism.