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