Unit 3.3
Scheduling

Presenter Notes

本节幻灯片

Process Behavior

Bursts of CPU usage alternate with periods of waiting for I/O.
(a) A CPU-bound process.
(b) An I/O-bound process.

Presenter Notes

几乎所有的进程的（磁盘）I/O请求或计算都是交替突发的，典型的，CPU不停顿的运行一段时间，然后发出一个系统调用以便读写文件。

Scheduling Algorithm Goals

Some goals of the scheduling algorithm under different circumstances.

Presenter Notes

进程调度所采用的算法是与整个系统的设计目标相一致的： 1.批处理系统：增加系统吞吐量和提高系统资源的利用率; 2.分时系统：保证每个分时用户能容忍的响应时间。 3.实时系统：保证对随机发生的外部事件做出实时响应。

Scheduling in Batch Systems

Batch Systems are usually non-preemptible.

  • First Come First Served
  • Shortest Job First
  • Shortest Remaining-time Next
  • High Response-ratio First

Presenter Notes

1.先来先服务 2.最短作业优先 3.最短剩余时间优先 4.高响应比优先

Parameters in scheduling algorithm

  • Arrival time / Finish time (point)
  • Waiting time / Executing time (period)
  • Turnaround time = Waiting time + Executing time = Finish time – Arrival time
  • Weighting turnaround time = Turnaround time / Executing time
  • Response ratio = Turnaround time / Executing time = 1 + Waiting time / Executing time

Presenter Notes

Scheduling in Interactive Systems

  • Round-robin scheduling
  • Priority scheduling
  • Multiple queues
  • Shortest process next
  • Guaranteed scheduling
  • Lottery scheduling
  • Fair-share scheduling

Presenter Notes

1.轮转调度 2.优先级调度 3.多级队列 4.最短进程优先 5.保证调度 6.彩票调度 7.公平分享调度

Priority Scheduling

A scheduling algorithm with four priority classes.

Presenter Notes

只要存在优先级为第四类的可运行进程，就按照轮转算法为每个进程运行一个时间片，此时不例会较低优先级的进程。若第四类进程为空，则按照轮转算法运行第三类进程。若第四类和第三类均为空，则按照轮转法运行第二类进程。如果不偶尔对优先级进行调整，则低优先级进程很可能会产生饥饿现象

Reference

  • Chapter 2: Processes and threads, Modern Operating Systems . Forth Edition, Andrew S. Tanenbaum

Presenter Notes