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

Normally the decision about which core an app will run on is made by the system. However, you can set the "affinity" for an application to a specific core to tell the OS to only run the app on that core. Normally this isn't a good idea, but there are some rare cases where it might make sense.

To do this in windows, use task manager, right click on the process, and choose "Set Affinity". You can do it programmatically in Windows using functions like SetThreadAffinityMask, SetProcessAffinityMask or SetThreadIdealProcessor.

ETA:

If you are interested in how the OS actually does the scheduling, you might want to check out these links:

Wikipedia article on context switching

Wikipedia article on scheduling

Scheduling in the linux kernel

With most modern OS's, the OS schedules a thread to execute on a core for a short slice of time. When the time slice expires, or the thread does an IO operation that causes it to voluntarily yield the core, the OS will schedule another thread to run on the core (if there are any threads ready to run). Exactly which thread is scheduled depends on the OS's scheduling algorithm.

The implementation details of exactly how the context switch occurs are CPU & OS dependent. It generally will involve a switch to kernel mode, the OS saving the state of the previous thread, loading the state of the new thread, then switching back to user mode and resuming the newly loaded thread. The context switching article I linked to above has a bit more detail about this.