Processes


Types of Schedulers cont…



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Lecture 2 Processes
Lecture 1 Introduction

Types of Schedulers cont…

  • Medium term Scheduler
    • Is part of the swapping function i.e. it is in charge of handling the swapped out-processes
    • It removes process from the memory e.g. suspended processes may be moved to the secondary storage (swapping) and make space for other processes
    • It reduces the degree of multiprogramming

Scheduling Algorithms

Scheduling Criteria

  • CPU Utilization – CPU is costly device, it must be kept as busy as possible
  • Throughput – refers to the number of processes completed in unit time
  • Turn around time – the time interval between the submission of the process and time of completion
  • Waiting time – amount of time a process has been waiting in the ready queue
  • Response time – time duration between the submission and first response

Scheduling Algorithms

  • A major division among scheduling algorithms is whether they support pre-emptive or non-pre-emptive scheduling discipline:
  • Pre-emptive scheduling
    • A scheduler may pre-empt a low priority running process anytime when a high priority process enters into a ready state
    • Pre-emptive scheduling is more useful in high priority process which requires immediate response e.g Real time system.
    • Round Robin scheduling, priority based scheduling or event driven scheduling and SRT
  • Non pre-emptive scheduling
    • Once a process enters the running state, it cannot be pre-empted until it completes its allocated time
    • Always processes a scheduled job to its completion.
    • First come First Served (FCFS) and Shortest Job First (SJF)

Scheduling Algorithms

  • First-Come First-Serve (FCFS)
  • Shortest-Job First (SJF)
  • Round Robin (RR)
  • Shortest Remaining Time Next (SRTN)
  • Priority Based Scheduling or Event Driven (ED) Scheduling
  • Multilevel Queue Scheduling (Research)
  • Multilevel Feedback Queue Scheduling (Research)

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