Issues With Priority

• Priority definition.
  • How to define a priority?
    • System policies: Charge, job quantum, I/O
    • External or internal definition.
• Pre-emptive vs. non-preemptive
  • When a process arrives, should we remove the running process? (e.g. SRTF + Priority)
• Starvation
  • When high priority jobs come at a frequency that a low priority job is blocked.
  • Solution: add “aging”.

Aging with Priority

• Each time a process is scheduled,
  • Increase the priority of each pending task by 1. (could even be periodic)
• Priority: defined at admission time only.
• In pre-emptive scheduling.
  • When a job is pre-empted, reset the priority.
• Guaranteed “No starvation”.