October 1, 2019


CPU scheduling is a process which allows one process to use the CPU while the execution of another process is on hold(in waiting state) due to unavailability of. CPU Scheduling Algorithms are used to assign the processes to the CPU. There are various CPU Scheduling Algorithms. Practice Problems based on. Avg. Wait = 0+8+14+15+24 = 61/5 = ms Avg. TAT = 8+14+15+24+27 = ms. Shortest Job First. P5. P2. P1. P4. CPU Scheduling Exercises. Problem 1.

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The Process Scheduler then alternately selects jobs from each queue and assigns them to the CPU based on the algorithm assigned to the queue. The interval from time of submission of the process to the time of completion of the process Wall clock time. Each process is assigned a priority.

To decide which process to execute first and which process to execute last to achieve maximum CPU utilisation, computer scientists have defined some algorithms, they are:. They make use of other existing algorithms to group and schedule jobs with common characteristics.

The processor is allocated to the job closest to completion but it can be preempted by a newer ready job with shorter time to completion. Dispatch Latency can be explained using the below figure:. There is a choice, however in circumstances 2 and 3.

In general CPU utilization and Throughput are maximized and other factors are reduced for proper optimization. Amount of time it takes from when a request was submitted until the first response is produced. Under non-preemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the nujericals state.

Operating System Scheduling algorithms

Non-preemptive algorithms are designed so that once a process enters the running state, it cannot be preempted until it completes its allotted time, whereas the preemptive scheduling is based on priority where a scheduler may preempt a low priority running process anytime when a high priority process enters into a ready state. Multiple-level queues are not an independent scheduling algorithm.


Therefore, the running task is interrupted for some time and resumed later when the priority task has finished its scjeduling. Priority can be decided based on memory requirements, time requirements schedulijg any other resource requirement. In this type of Scheduling, the tasks are usually assigned with priorities.

The sum of the periods spent waiting numeficals the ready queue amount numericalx time a process has been waiting in the ready queue to acquire get control on the CPU. Switching context Switching to user mode Jumping to the proper location in the user program to restart that program from where it left last time. When Scheduling takes place only under circumstances 1 and 4, we say the scheduling scheme is non-preemptive ; otherwise the scheduling scheme is preemptive.

Remember, it is the time till the first response and not the completion of process execution final response. It is the average number of processes residing in the ready queue waiting for their turn to get into the CPU. When a process switches from the running state to the ready state for example, when an interrupt occurs. Dispatch Latency can be explained using the below figure: Process with highest priority is to be executed first and so on.

Another component involved in the CPU scheduling function is the Dispatcher. The scheduler selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.

It is the total number schedulnig processes completed per unit time or rather say total amount of work done in a unit of time. At times it is necessary to run a certain task that has a higher priority before another task although it is running.


We will be discussing all the scheduling algorithms, one by one, in detail in the next tutorials.

Priority scheduling is a non-preemptive algorithm and one of the numerifals common scheduling algorithms in batch systems. Operating System Scheduling algorithms Advertisements. It is the only method that can be used on certain hardware platforms, because It does not require the special hardware for example: In circumstances 1 and 4, there is no choice in terms of scheduling.

CPU Scheduling in Operating System | Studytonight

The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. A new process if one exists in the ready queue must be selected for execution. Numeircals are many different criterias to check when considering the “best” scheduling algorithm, they are:.

When a process terminates.

What is CPU Scheduling?

The aim of CPU scheduling is to make the system efficient, fast and fair. It is the amount of time taken to execute a particular process, i. The dispatcher should be as fast as possible, given that it is invoked during every process switch.

These algorithms are either non-preemptive or preemptive. What is CPU Scheduling? The time taken by the dispatcher to stop one process and start another process is known as the Dispatch Latency. This scheduling method is used by the Microsoft Windows 3. Whenever the CPU becomes idle, the operating system must select one of the processes in the ready queue to be executed.

Once a process is executed for a given time period, it is preempted and other process executes for a given time period.