How do SMP cores, processes, and threads work together exactly?

Question

On a single core CPU, each process runs in the OS, and the CPU jumps around from one process to another to best utilize itself. A process can have many threads, in which cas

Answer 1

Yes, threads and processes can run concurrently on multi-core CPUs, so this works as you describe (regardless of how you create those threads and processes, OpenMP or otherwise). A single process or thread only runs on a single core at a time. If there are more threads requesting CPU time than available cores (generally the case), the operating system scheduler will move threads on and off cores as needed.

The reason why single-threaded processes run on more than one CPU or core is related to your operating system, and not specifically any feature of the hardware. Some operating systems have no sense of "thread affinity" - they don't care what processor a thread is running on - so when time comes to re-evaluate what resources are being used (several times a second, at least), they'll move a thread/process from one core/CPU to another. Other than causing cache misses, this generally doesn't affect the performance of your process.