How to control which core a process runs on?

Solution 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).

Solution 2:

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.