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Summary of Section 4.1 - Principles of Scheduling

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Long-Term vs. Short-Term Scheduling

• Long-term scheduling controls when a process enters the ready list (RL) to compete for CPU time.
• Short-term scheduling determines which process from the ready list should execute next.

Processes can enter the ready list from:

1. New processes list → Subject to long-term scheduling.
2. Suspended list → Subject to long-term scheduling.
3. Waiting list → Subject to short-term scheduling.
4. CPU (after execution time expires) → Subject to short-term scheduling.

Preemptive vs. Non-Preemptive Scheduling

• Non-preemptive scheduling — Here the resource can’t be taken from a process until the process completes execution. The switching of resources occurs when the running process terminates and moves to a waiting state.
• Preemptive scheduling allows the OS to interrupt a process and switch to another when:
  • A new process enters the ready list.
  • A previously blocked/suspended process re-enters the ready list.
  • The OS periodically interrupts a process to ensure fairness.

Priority in Short-Term Scheduling

• Priority values determine which process executes next.
• Priority can be static (fixed at creation) or dynamic (based on multiple factors).

Common factors affecting priority:

1. Arrival time – When the process enters the ready list.
2. Departure time – When the process exits due to blocking or termination.
3. Attained CPU time – The CPU time used by a process since arrival.