CPU scheduling

What happens when an interrupt occurs in CPU scheduling?

• CPU stops current code execution.

• All data is stored.

• CPU switches to kernel mode.

• Two mode switches: to OS and back to user mode.

What does CPU scheduling decide?

• When a process needs to change state.

• Involves long-term, medium-term, and short-term scheduling.

What is long-term scheduling?

• Adds new processes to the execution pool.

• Controls the degree of multiprogramming.

What is medium-term scheduling?

• Manages swapping between main memory and secondary storage.

• Decides how much CPU to allocate to processes.

What is short-term scheduling?

• Picks which ready process to run next on the CPU.

• Also known as the dispatcher.

What criteria do short-term scheduling algorithms use?

• Optimizing user or system behavior.

• Including throughput, wait time, and fairness.

What's the difference between preemptive and non-preemptive scheduling?

• Non-preemptive: Process runs until it blocks or finishes.

• Preemptive: OS can interrupt and switch running processes.

How does FCFS scheduling work?

• The oldest waiting process is assigned to the CPU next.

• Non-preemptive with no timer interrupts.

What is Round Robin scheduling?

• Processes get equal CPU time slices.

• Preemptive, with time slices dictating switches.

What is Shortest Process Next scheduling?

Prioritizes process with shortest expected duration.

Non-preemptive; longer tasks may face starvation.

How does Shortest Remaining Time scheduling work?

Chooses process with least time left to completion.

Preemptive; can interrupt longer tasks for new, shorter ones.

What is the Highest Response Ratio Next scheduling strategy?

Selects process with highest wait-to-service time ratio.

Non-preemptive; accounts for both service time and wait time.

What are multilevel feedback queues in scheduling?

• Processes are placed in priority queues.

• I/O bound processes move up, CPU bound processes move down.

• Uses preemptive FCFS within each queue.