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Process Scheduling in Operating System
Scheduling QueueJob Queue :A process when enters a system is put into a job queue . | Ready Queue: The processes residing in main memory and ready for execution are put in the ready queue. | Device Queue: The processes waiting for a particular I/O device are put into the device queue. |
Scheduling Criteria1. CPU Utilization: It should be maximum. 40% minimum- 90% maximum. | 2. Throughput: Number of processes that are completed per unit time are called throughput. It should be maximum. | 3. Turnaround Time: The interval from time of submission of process to time of completion. Turnaround time= period spent waiting+ready queue time+execution+I/O interrupt time. It should be minimum. |
1. First Come First Serve (FCFS)
Priority Scheduling Diagram
5. Multilevel Queue Scheduling
| | Scheduling Queue Diagram
Medium Term scheduler Diagram
SCHEDULING ALGORITHMScheduling Algorithm decides which process should the CPU be allocated to. There are six scheduling algorithms. |
First Come First Serve Shortest Job FirstThe process that requests for the CPU first, gets the access first. | Each process has the length of the next CPU burst. | FIFO queue is used in handling the process. | The process with the smallest next CPU burst gets access to the process. | Long waiting time for the next processes. | Comparitively less waiting time for next process. | It is a non-preemptive algorithm. | It can be premptive or non-preemptive algorithm. |
Priority Scheduling Round Robin SchedulingEach process has a priority. | There is a fixed time quantum. | CPU allocated to the process with higher priority. | Ready queue is treated as a circular queue and CPU is allocated to the First process for specific time quantum. | Problem: Starvation of low priority process.Solution: AGING | Problem: If time quantum is too large, algorithm works as FCFS. | It can be preemptive or non-preemptive | It is preemptive. |
Multilevel Queue Multilevel Feedback QueueReady queue is divided into: Foreground (interactive) process and Background (batch) process. | Allows the process from one queue to move to the next queue. | Foreground implements Round Robin Scheduling. and Background implements FCFS. | Here processes are separated according to their CPU burst. |
| | SchedulerLong Term Scheduler: Also known as job scheduler selects process from disk and puts into memory. | Short Term Scheduler: Also known as CPU scheduler select process from memory and allocates a CPU to it. | Medium Term Scheduler: It is used to remove a process from and reduce degree of multi-programing .Later it can be re-introduced from point where it was left . This is called as swapping. |
Scheduling Criteria4. Waiting Time: The time for which the process has to wait in the ready queue is waiting time. It should be minimum. | 5. Response Time: Time taken to respond to a process is response time. It should be minimum. |
2. Shortest Job First (SJF)
4. Round Robin Scheduling Diagram
6. Multilevel Feedback Queue Scheduling
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