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

As others have mentioned, it's controlled by the operating system. Depending on the OS, it may or may not provide you with system calls that allow you to affect what core a given process executes on. However, you should usually just let the OS do the default behavior. If you have a 4-core system with 37 processes running, and 34 of those processes are sleeping, it's going to schedule the remaining 3 active processes onto separate cores.

You'll likely only see a speed boost on playing with core affinities in very specialized multithreaded applications. For example, suppose you have a system with 2 dual-core processors. Suppose you have an application with 3 threads, and two of threads operate heavily on the same set of data, whereas the third thread uses a different set of data. In this case, you would benefit the most by having the two threads which interact on the same processor and the third thread on the other processor, since then they can share a cache. The OS has no idea what memory each thread needs to access, so it may not allocate threads to cores appropriately.

If you're interested in how the operating system, read up on scheduling. The nitty gritty details of multiprocessing on x86 can be found in the Intel 64 and IA-32 Architectures Software Developer's Manuals. Volume 3A, Chapters 7 and 8 contain relevant information, but bear in mind these manuals are extremely technical.