CPU Scheduling Algorithms
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The document discusses various CPU scheduling algorithms including first come first served, shortest job first, priority, and round robin. It describes the basic concepts of CPU scheduling and criteria for evaluating algorithms. Implementation details are provided for shortest job first, priority, and round robin scheduling in C++.
![Implementation in C++
Class: cpuschedule
Attributes:
o n – number of processes
o Bu[ ] – Array to store Burst Time
o A[ ] – Array to store Arrival Time
o Wt[ ] – Array to store Waiting Time
o Twt – Total Waiting Time
o Awt – Average Waiting Time
CPU Scheduling Algorithms 9](https://image.slidesharecdn.com/final-130402232110-phpapp02/85/CPU-Scheduling-Algorithms-9-320.jpg)
![Round Robin
Process Burst Time
P1 24
P2 3
P3 3
Quantum time = 4 milliseconds
The Gantt chart is:
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30
Average waiting time = {[0+(10-4)]+4+7}/3 = 5.6
CPU Scheduling Algorithms 19](https://image.slidesharecdn.com/final-130402232110-phpapp02/85/CPU-Scheduling-Algorithms-19-320.jpg)
CPU Scheduling Algorithms
- 1. CPU Scheduling Algorithms Group No. 2 1
- 2. Overview Basic Concepts Scheduling Criteria Scheduling Algorithms Implementation in C++ Demonstration Involvement of Operating System CPU Scheduling Algorithms 2
- 3. Basic Concepts Main objective of multiprogramming is to keep on running processes all the time for maximum CPU utilization. Scheduling is fundamental function of OS. The task of selecting the processes in memory that are ready to execute, and allocating them to the CPU is performed by the CPU Scheduler. CPU Scheduling Algorithms 3
- 4. CPU Scheduler CPU scheduling decisions may take place when a process: o 1. Switches from running to waiting state o 2. Switches from running to ready state o 3. Switches from waiting to ready o 4. Terminates Scheduling under 1 and 4 is non preemptive. All other scheduling is preemptive. CPU Scheduling Algorithms 4
- 5. CONT… CPU Scheduler Nonpreemptive Once a process is allocated the CPU, it does not leave unless: o it has to wait, e.g., for I/O request o it terminates Preemptive o OS can force (preempt) a process from CPU at anytime o E.g., to allocate CPU to another higher-priority process CPU Scheduling Algorithms 5
- 6. Scheduling Criteria CPU utilization: keep the CPU as busy as possible ◦ Maximize Throughput: No of processes that complete their execution per time unit ◦ Maximize Turnaround time: amount of time to execute a particular process (time from submission to termination) ◦ Minimize CPU Scheduling Algorithms 6
- 7. CONT… Scheduling Criteria Waiting time: amount of time a process has been waiting in the ready queue (sum of time waiting in ready queue) o Minimize Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) o Minimize CPU Scheduling Algorithms 7
- 8. Scheduling Algorithms First Come, First Served Shortest Job First Priority Round Robin CPU Scheduling Algorithms 8
- 9. Implementation in C++ Class: cpuschedule Attributes: o n – number of processes o Bu[ ] – Array to store Burst Time o A[ ] – Array to store Arrival Time o Wt[ ] – Array to store Waiting Time o Twt – Total Waiting Time o Awt – Average Waiting Time CPU Scheduling Algorithms 9
- 10. CONT… Implementation in C++ Operations: o Getdata() – To get number of processes and Burst Times from the user o Fcfs() – First Come, First Served Algorithm o Sjf() – Shortest Job First (normal) Algorithm o SjfP() – Shortest Job First (Preemption) Algorithm o SjfNp() – Shortest Job First (non preemption) Algorithm o Priority() – Priority Algorithm o RoundRobin() – Round Robin Algorithm CPU Scheduling Algorithms 10
- 11. First Come, First Served Process Burst Time P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P1 P2 P3 0 24 27 30 Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 CPU Scheduling Algorithms 11
- 12. First Come First CONT… Served Suppose that the processes arrive in the order : P2 , P3 , P1 (P1:24,P2:3,P3:3) The Gantt chart for the schedule is: P2 P3 P1 0 3 6 30 Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case Convoy effect - short process behind long process CPU Scheduling Algorithms 12
- 13. Shortest Job First Normal SJF Process Burst Time P1 7 P2 3 P3 4 The Gantt Chart for SJF (Normal) is: P2 P3 P1 0 3 7 14 Average waiting time = (0 + 3 + 7)/3 = 3.33 CPU Scheduling Algorithms 13
- 14. Shortest Job First CONT… Non-Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 The Gantt Chart for SJF (non-preemptive) is: P1 P3 P2 P4 0 2 4 5 7 8 12 16 Average waiting time = (0 + 6 + 3 + 7)/4 = 4 CPU Scheduling Algorithms 14
- 15. Shortest Job First CONT… Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 The Gantt Chart for SJF (preemptive) is: P1 P2 P3 P2 P4 P1 0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)/4 = 3 CPU Scheduling Algorithms 15
- 16. Shortest Job First CONT… Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: o Non-Preemptive: once CPU given to the process it cannot be preempted until completes its CPU burst. o Preemptive: if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF). SJF is optimal: gives minimum average waiting time for a given set of processes. CPU Scheduling Algorithms 16
- 17. Priority Process Burst Time Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 Gantt Chart P2 P5 P1 P3 P4 0 1 6 16 18 19 Average waiting time = (6 + 0 + 16 + 18 + 1)/5 = 8.2 CPU Scheduling Algorithms 17
- 18. Priority CONT… A priority number (integer) is associated with each process. Lager the CPU burst lower the priority. The CPU is allocated to the process with the highest priority (smallest integer ≡ highest priority) Starvation (Infinity blocking): low priority processes may never execute. Aging: as time progresses increase the priority of the process. CPU Scheduling Algorithms 18
- 19. Round Robin Process Burst Time P1 24 P2 3 P3 3 Quantum time = 4 milliseconds The Gantt chart is: P1 P2 P3 P1 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30 Average waiting time = {[0+(10-4)]+4+7}/3 = 5.6 CPU Scheduling Algorithms 19
- 20. Round Robin CONT… Typically, higher average turnaround than SJF, but better response 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. Performance o q large ⇒ FCFS o q small ⇒ q must be large with respect to context switch, otherwise overhead is too high CPU Scheduling Algorithms 20
- 21. Involvement of OS Source Code (.c) Compiler Conversion (.i , .o) Executable (.exe) Microsoft (load executable Micro-kernel Windows directly to memory) Memory CPU Scheduling Algorithms Execut e CPU Scheduling Algorithms 21
- 22. CPU Scheduling Algorithms Group No. 2 Group No. 2 22
Editor's Notes
- #18: Sonali C. UDIT CPU Scheduling Aging - Gradually increasing the priority of process that waits in the system for long time.