Basic Concepts

CPU-I/O Burst

CPU Scheduler

• Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them
• CPU scheduling decisions may take place when a process:
  1. Switches from running to waiting state (non-preemptive)
  2. Switches from running to ready state (preemptive)
  3. Switches from waiting to ready (preemptive)
  4. Terminates (non-preemptive)

Preemptive and Nonpreemptive Scheduling

• Preemptive scheduling
  • Higher responsiveness
  • Higher context switch overheads
  • Must deal with race conditions
• Cooperative scheduling (Nonpreemptive)
  • Easy to implement
  • Poor responsiveness
  • Ill-behaved processes will take over the system

Dispatcher

• Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:

  • switching context
  • switching to user mode
  • jumping to the proper location in the user program to restart that program

• Dispatch latency (the context-switch overhead):

  The time it takes for the dispatcher to stop one process and start another running

  

Scheduling Criteria

• Max CPU utilization (CPU使用率）
• Max throughput (單位時間內完成的 process 數量)