Process Scheduling

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Process Scheduling

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Process Scheduling

  1. 1. Process Scheduling Abhishek Nagar Process Scheduling -Abhishek Nagar
  2. 2. Agenda Introduction The Basics (Keywords) Scheduling Policies Scheduling Algorithms First Come First Served Shortest Job First Round Robin Priority Process Scheduling -Abhishek Nagar
  3. 3. Introduction In multiprogramming systems, when there is more than one ready process, the operating system must decide which one to activate. The decision is made by the part of the operating system called the scheduler, using a scheduling algorithm. The scheduler is concerned with deciding policy, not providing a mechanism Process Scheduling -Abhishek Nagar
  4. 4. The Basics (Keywords) Process CPU Burst (Usage Time) Turnaround Time Waiting Time Time Quantum (time slice) Process Scheduling -Abhishek Nagar
  5. 5. Scheduling Policies Preemptive Non Preemptive Process Scheduling -Abhishek Nagar
  6. 6. First Come First Served FCFS, also known as First-In-First-Out (FIFO), is the simplest scheduling policy. Arriving jobs are inserted into the tail (rear) of the ready queue and the process to be executed next is removed from the head (front) of the queue. Process Scheduling -Abhishek Nagar
  7. 7. FCFS - Example Process CPU Turnaround Waiting Burst Time Time A 10 10 0 Process B 1 11 10 C 3 14 11 D 4 18 14 13.25 8.75 AVERAGE D C B A 5 10 15 20 25 Time Process Scheduling -Abhishek Nagar
  8. 8. Shortest Job First SJF policy selects the job with the shortest (expected) processing time first. Shorter jobs are always executed before long jobs. long running jobs may starve, because the CPU has a steady supply of short jobs. Process Scheduling -Abhishek Nagar
  9. 9. Shortest Job First - Example Process CPU Turnaround Waiting Burst Time Time Process A 10 18 8 B 1 1 0 C 3 4 1 D 4 8 4 7.75 3.25 AVERAGE D C B A 5 10 15 20 25 Time Process Scheduling -Abhishek Nagar
  10. 10. Round Robin RR reduces the penalty that short jobs suffer with FCFS by preempting running jobs periodically. The CPU suspends the current job when the reserved quantum (time-slice) is exhausted. The job is then put at the end of the ready queue if not yet completed. Process Scheduling -Abhishek Nagar
  11. 11. Round Robin - example Time Slice = 1 Process Process CPU Turnaround Waiting Burst Time Time A 10 18 8 B 1 2 1 C 3 9 6 D 4 12 6 D 10.25 5.75 AVERAGE C B A 5 10 15 20 25 Time Ready Queue Status : A B C D A C D A C D A D A Process Scheduling -Abhishek Nagar
  12. 12. Priority Each process is assigned a priority. The ready list contains an entry for each process ordered by its priority. The process at the beginning of the list (highest priority) is picked first. A variation of this scheme allows preemption of the current process when a higher priority process arrives. Process Scheduling -Abhishek Nagar
  13. 13. Priority - Example Process PriorityCPU Turnaround Waiting Burst Time Time Process A 4 10 18 8 B 3 1 8 7 C 2 3 7 4 D 1 4 4 0 9.25 4.75 AVERAGE D C B A 5 10 15 20 25 Time Process Scheduling -Abhishek Nagar
  14. 14. Thank You ! Questions and Comments Welcome! Abhishek Nagar abhishek@nagarji.net Process Scheduling Simulation from gaia.ecs.csus.edu Process Scheduling -Abhishek Nagar

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