CPU Scheduling in Operating Systems: Types, Concepts, and Algorithms, Study notes of Computer Science

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Operating SystemsOperating Systems

CMPSC 473 CMPSC 473

CPU Scheduling CPU Scheduling

February 12, 2008 - Lecture February 12, 2008 - Lecture 88

Instructor: Trent Jaeger Instructor: Trent Jaeger

• Last class:
  • Threads
• Today:
  • CPU Scheduling

Resources Types

• Memory : Allocate portion of finite resource
  • Virtual memory tries to make this appear infinite
  • Physical resources are limited
• I/O : Allocate portion of finite resource and time with

resource

• Store information on disk
• A time slot to store that information
• CPU : Allocate time slot with resource
• A time slot to run instructions
• We will focus on CPU scheduling in the section

CPU Scheduling Examples

• Single process view

• GUI request
  • Click on the mouse
• Scientific computation
  • Long-running, but want to complete ASAP

• System view

• Get as many tasks done as quickly as possible
• Minimize waiting time for processes
• Utilize CPU fully

Scheduling Problem

• Choose the ready/running process to run at any time
  • Maximize “performance”
• Model/estimate “performance” as a function
  • System performance of scheduling each process
    • f(process) = y
  • What are some choices for f(process)?
• Choose the process with the best y
  • Estimating overall performance is intractable
    • E.g., scheduling so all tasks are completed as soon as possible

Scheduling Concepts

Preemptive vs Non-

preemptive

• Can we reschedule a process that is actively running?
  • If so, we have a preemptive scheduler
  • If not, we have a non-preemptive scheduler
• Suppose a process becomes ready
  • E.g., new process is created or it is no longer waiting
• It may be better to schedule this process
  • So, we preempt the running process
• In what ways could the new process be better?

Bursts

• A process runs in CPU and I/O Bursts

• Run instructions (CPU Burst)
• Wait for I/O (I/O Burst)

• Scheduling is aided by knowing the length of

these bursts

• More later…

CPU Burst Duration

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 – time it takes for the dispatcher to stop

one process and start another running

Scheduling Criteria

• Utilization/efficiency: keep the CPU busy 100% of the time with useful work
• Throughput: maximize the number of jobs processed per hour.
• Turnaround time: from the time of submission to the time of completion.
• Waiting time: Sum of times spent (in Ready qqueue) waiting to be scheduled on the CPU.
• Response Time: time from submission till the first response is produced (mainly for interactive jobs)
• Fairness: make sure each process gets a fair share of the CPU

Scheduling Algorithms

First-Come, First-Served (FCFS) Process Burst Time P 1 24 P 2 3 P 3 3

• Suppose that the processes arrive in the order: P 1 , P 2 , P 3 The Gantt Chart for the schedule is:
• Waiting time for P 1 = 0; P 2 = 24; P 3 = 27
• Average waiting time: (0 + 24 + 27)/3 = 17 P 1 P 2 P 3 0 24 27 30

Reducing Waiting Time

Suppose that the processes arrive in the order P 2 , P 3 , P 1

• The Gantt chart for the schedule is:
• Waiting time for P 1 = 6 ; P 2 = 0 ; P 3 = 3
• Average waiting time: (6 + 0 + 3)/3 = 3
• Much better than previous case
• Convoy effect short process behind long process P 2 P 3 P 1 0 3 6 30