CPU Scheduling: Algorithms and Systems, Study notes of Computer Science

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Operating SystemsOperating Systems

CMPSC 473 CMPSC 473

CPU Scheduling CPU Scheduling

February 14, 2008 - Lecture February 14, 2008 - Lecture 99

Instructor: Trent Jaeger Instructor: Trent Jaeger

• Last class:
  • CPU Scheduling
• Today:
  • CPU Scheduling Algorithms and Systems

Priority Scheduling

• Each process is given a certain priority

“value”.

• Always schedule the process with the highest

priority.

P5 5 2

P4 1 5

P3 2 4

P2 1 1

P1 10 3

Duration(s) Priority

Gantt Chart for Priority Scheduling

P 0 1 P 6 P 16 P 18 P 19

Round Robin (RR)

• Each process gets a small unit of CPU time ( time

quantum )

• Usually 10-100 milliseconds
• After this time has elapsed, the process is preempted and added to the end of the ready queue
• Approach
• If there are n processes in the ready queue and the time quantum is q
• Then each process gets 1/ n of the CPU time
• In chunks of at most q time units at once.
• No process waits more than ( n -1) q time units

An example of Round Robin

P3 0 7

P2 0 3

P1 0 24

Job length (s) Arrival Time (s) Time Quantum = 4 s P (^0 ) P 7 P 11 P 15 P 18 P 22 P 26 P 30 P 34

RR Time Quantum

• For the considered example, if time quantum

size drops to 2s from 4s, the number of

context switches increases to ????

• But context switches are not free!

– Saving/restoring registers

– Switching address spaces

– Indirect costs (cache pollution)

Scheduling Desirables

• SJF
  • Minimize waiting time
    • Requires estimate of CPU bursts
• Round robin
  • Share CPU via time quanta
    • If burst turns out to be “too long”
• Priorities
  • Some processes are more important
  • Priorities enable composition of “importance” factors
• No single best approach -- now what?

Round-Robin with Priority

Priority Levels

What is the problem?

• With fixed priorities, processes lower in the

priority level can get starved out!

• In general, you employ a mechanism to “age”

the priority of processes.

Example of Multilevel

Feedback Queue

• Three queues:
  • Q 0 – RR with time quantum 8 milliseconds
  • Q 1 – RR time quantum 16 milliseconds
  • Q 2 – FCFS
• Scheduling
  • A new job enters queue Q 0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1.
  • At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.

Multilevel Feedback Queues

Solaris 2 Scheduling

Solaris Dispatch Table