[OS]Process Scheduling

Multi-programming

• Multiple processes exist in the system
• Select a process to allocate resources
  • Scheduling
• Resource Management
  • time sharing
    • Process scheduling
  • space sharing
    • memory

Purpose of Scheduling

• Improves system performance
• System Performance index
  • response time
  • throughput
  • resource utilization

Scheduling Criteria

• I/O-bounded or compute-bounded
• Batch system or interactive system
• process urgency
• process priority
• process total service time

CPU burst vs I/O burst

• Performing the Process
  • Use CPU + I/O
• compute-bounded
  • CPU burst > I/O burst
• I/O-bunded
  • I/O burst > CPU burst

Scheduling Level

• Long-term scheduling
  • Job scheduling
• Mid-term scheduling
  • Memory allocation
• Short-term scheduling
  • Process scheduling

Long-term Scheduling

• Job scheduling
• Multiprogramming degree control
  • Controlling the number of processes in the system

Mid-term Scheduling

• Memory allocation
  • Intermediate-level scheduling
  • Swapping(swap-in/swap-out)

Short-term Scheduling

• Process scheduling
  • Low-level scheduling
  • Determine the process by which to allocate processors
• Most frequent occurrence

Scheduling policy

• Preemptive scheduling, Non-preemptive scheduling
• Priority

Non-preemptive scheduling

• system call, I/O, Etc.
• Low context switch overhead
• Frequent priority reversals, increased average response time

Preemptive scheduling

• High context switch overhead
• Time-sharing system, real-time system

Priority

• Staic priority
• Dynamic priority

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Reference

Process Scheduling