Process Scheduling¶

• Multiprogramming : maximize CPU utilization

• Time sharing: users can interact with each program while it is running

Process Scheduling Queues¶

• Job queue : all processes
• Ready queue : all ready to executte
• Device queue : waiting for an IO device

Schedulers¶

• Short-term scheduler (CPU scheduler) : (Ready state \(\to\) Run state)
• Long-term scheduler (job scheduler) : (New state \(\to\) Ready state)
• Medium-term scheduler : swapped in/out memory (Ready state \(\to\) wait state) (swap)

Long-term scheduler¶

• Control degree of multiprogramming
• Execute less frequently (once several minutes)
• Select a good mix of CPU-bound & IO-bound
• UNIX/NT : no long-term scheduler
  • created process placed in memory for short-term scheduler
  • Multiprogramming degree is bounded by hardware limitations

Short-term scheduler¶

• Execute quite frequently (once per 100ms)
• Must be efficient

Medium-term scheduler¶

• swap out : remove from memory

• swap in : fetch from swap

• improve process mix, free up memory

• Related to virtual memory