Comment by throwup238

> What goes into ensuring that a program is actually realtime?

Realtime mostly means predictable runtime for code. As long as its predictable, you can scale the CPU/microcontroller to fit your demands or optimize your code to fit the constraints. It’s about making sure your code can always respond in time to hardware inputs, timers, and other interrupts.

Generally the Linux kernel’s scheduling makes the system very unpredictable. RT linux tries to address that along with several other subsystems. On embedded CPUs this usually means disabling advanced features like cache, branch prediction, and speculative execution (although I don’t remember if RT handles that part since its very vendor specific).