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 communication mechanisms.

I also understand context switching. That is, with only once CPU, the operating system schedules time for each process (and there are tons of scheduling algorithms out there) and thereby we achieve running multiple processes simultaneously.

And now that we have multi-core processors (or multi-processor computers), we could have two processes running simultaneously on two separate cores.

My question is about the last scenario: how does the kernel control which core a process runs on? Which system calls (in Linux, or even Windows) schedule a process on a specific core?

The reason I’m asking: I’m working on a project for school where we are to explore a recent topic in computing – and I chose multi-core architectures. There seems to be a lot of material on how to program in that kind of environment (how to watch for deadlock or race conditions) but not much on controlling the individual cores themselves. I would love to be able to write a few demonstration programs and present some assembly instructions or C code to the effect of ‘See, I am running an infinite loop on the 2nd core, look at the spike in CPU utilization for that specific core‘.

Any code examples? Or tutorials?

edit: For clarification – many people have said that this is the purpose of the OS, and that one should let the OS take care of this. I completely agree! But then what I’m asking (or trying to get a feel for) is what the operating system actually does to do this. Not the scheduling algorithm, but more ‘once a core is chosen, what instructions must be executed to have that core start fetching instructions?’

Answer 1

As others have mentioned, processor affinity is Operating System specific. If you want to do this outside the confines of the operating system, you’re in for a lot of fun, and by that I mean pain.

That said, others have mentioned SetProcessAffinityMask for Win32. Nobody has mentioned the Linux kernel way to set processor affinity, and so I shall. You need to use the sched_setaffinity(2) system call. Here’s a nice tutorial on how.

The command-line wrapper for this system call is taskset(1). e.g.
taskset -c 2,3 perf stat awk 'BEGIN{for(i=0;i<100000000;i++){}}' restricts that perf-stat of a busy-loop to running on either of core 2 or 3 (still allowing it to migrate between cores, but only between those two).