| Updated: April 19, 2023 |
Now, suppose that two threads are capable of using the CPU and have the exact same priority. Let's assume that one of the threads is currently using the CPU. We'll examine the rules that the kernel uses to decide when to context-switch in this case.
(Of course, this entire discussion really applies only to threads at the same priority—the instant that a higher-priority thread is ready to use the CPU it gets it; that's the whole point of having priorities in a realtime operating system.)
The two main scheduling policies (policies) that the microkernel understands are Round Robin (or just “RR”) and FIFO (First-In, First-Out). (There's also sporadic scheduling, but it's beyond the scope of this book; see “Sporadic scheduling” in the QNX Neutrino Microkernel chapter of the System Architecture guide.)