How to control which core a process runs on?

Question

I can understand how one can write a program that uses multiple processes or threads: fork() a new process and use IPC, or create multiple threads and use those sorts of com

Answer 1

Nothing tells core "now start running this process".

The core does not see process, it only knows about executable code and various running levels and associated limitations to instructions that can be executed.

When computer boots, for sake of simplicity only one core/processor is active and actually runs any code. Then if OS is MultiProcessor capable, it activates other cores with some system specific instruction, other cores most likely pick up from exactly same spot as other core and run from there.

So what scheduler does is it looks through OS internal structures (task/process/thread queue) and picks one and marks it as running at its core. Then other scheduler instances running on other cores won't touch it until the task is in waiting state again (and not marked as pinned to specific core). After task is marked as running, scheduler executes switch to userland with task resuming at the point it was previously suspended.

Technically there is nothing whatsoever stopping cores from running exact same code at exact same time (and many unlocked functions do), but unless code is written to expect that, it will probably piss all over itself.

Scenario goes weirder with more exotic memory models (above assumes "usual" linear single working memory space) where cores don't necessarily all see same memory and there may be requirements on fetching code from other core's clutches, but it's much easier handled by simply keeping task pinned to core (AFAIK Sony PS3 architecture with SPU's is like that).