Lecture Outline

Aims and Objectives
The Life-Cycle of a Process (Process State Transitions)
Process Scheduling
Scheduling Objectives
Scheduling and Performance Criteria

• Life-Cycle of a Process
  

• Why Scheduling is important
  

• Different levels of Scheduling
  

• Scheduling Objectives
  

• Scheduling Criteria
  

The Life-Cycle of a Process (Process State Transitions)

• Policies and Mechanisms of selecting a runnable process to become the current process.

• Important to maintain system balance.

  

• Schedulers are processes. Need CPU to run. Scheduler tends to have highest priority in system, so that when current process is interrupted, scheduler becomes best process to run.

• Be Fair

• Maximize throughput

• Maximize number of users receiving acceptible response times.

• Be predictable

• Balance resource use

• Avoid indefinite postponement

• Enforce Priorities

• Give preference to processes holding key resources

• Give better service to processes that have desirable behaviour patterns

• Degrade gracefully under heavy loads

• CPU Utilisation - is the process CPU-bound or I/O-bound?

• Throughput - How many processes are executed in a unit of time?

• Turnaround time - how long does it take for a program to exit the system from when it is submitted?

• Waiting Time - how much time does the process spend waiting for the CPU in total?

• Response Time - How long does it take for the system to respond to an interactive request?

• The above should be prioritised, and a scheduling algorithm which satisfies the criteria should be used.

Summary

• Seen typical behaviour patterns displayed by processes during execution

• Organisation of different level schedulaers to maintain system balance and optimise resource utilisation

• What objectives a scheduler should achieve

• Questions to be answered to choose appropriate scheduling algorithm

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