CPU scheduling in the operating system

CPU scheduling in the operating system

CPU or central processing unit scheduling is the method used to schedule instructions and processes that desire to employ time of CPU. It permits one instruction or process which is known as P1 to use the time of CPU by perhaps placing another process called P2 that is in waiting or on hold queue for the time when P2 may be in a queue or waiting for an extra resource such as locked file or input etc.

One can use CPU scheduling in the operating system to create an effective and efficient application of the Central Processing Unit to enhance the CPU usefulness and to minimize and optimize usage of time.

If the Central Processing Unit gets idle or lazy, it may select a process from the ready queue 8using various algorithms to intelligently select the process that requires to be executed and implemented next.

The need for CPU Scheduling in an operating system

A method or process in the operating system requires scheduling in the operating system is given below:

1. Data or File Resources
2. Input-Output time
3. Central Processing Unit time

whereas in a process system much Central Processing Unit time can be exhausted, from the time when the present process might have to stay for locked or input resources, etc. This is the reason behind the waste time of the CPU. To overcome this situation, we can apply multi-programming or multi-tasking systems, that support method or process change if another process or method is currently running state is expecting for an input-output or resource.

In these kinds of cases, any process or instruction from the prepared queue is choose based on a pre-described logic, this is happened by the scheduler based on the short term.

Non-Preemptive and Preemptive Scheduling

1. Non-Preemptive Scheduling

In this kind of CPU scheduling, if instruction or process arrives at the CPU and gets time for processing. The instruction or process will maintain on implementing until it has ended or must effectively go to the state of waiting as it requires a resource that is protected by another parallel process. The example of non-preemptive scheduling is the shortest job first (SJF).

2. Preemptive Scheduling

In this kind of CPU scheduling, if instruction or process arrives the Central Processing Unit and gets the time of processing. It can get changed by any other instruction or process that is having higher priority.

Importance must force that may be allocated to each process or dynamic priority of time-based. An example of preemptive CPU scheduling is the SRTF algorithm. 

Criteria for Scheduling

 Central Processing Unit application maintains the CPU much busy. Some criteria for CPU scheduling are given below:

1. Throughput – throughput of the processes can be defined as, the completion of their execution per unit time.
2. Turnaround time – it is a value of time used to perform a process.
3. Waiting time – it is an amount of duration for which a process has been expecting in the queue of ready.
4. Response time – it is the value of time which takes since when demand or request was presented until the 1^st reply is generated, not taking about output.

Multiple-Processor CPU Scheduling

Central Processing Unit scheduling in the operating system is more complicated at the time of multiple availabilities of CPUs.

 If we Assume F type of Homogeneous instructions or processors within a unit of a multiprocessor. F is a UMA that is Uniform memory access.

If we talk about Load sharing, then we use the common queue which is ready for F Symmetric. every instruction checks for ready queue

In the case of Asymmetric multiprocessing, there is only one kind of processor retrieves the data structures of the system by easing the requirement for the protection of data sharing.

Schedulers in Operating System

There are three types of characteristics in schedulers in the operating system.  Those are mentioned below:

1. Schedulers in Operating System are very special types of tools or software.
2. Schedulers can aid in arranging and scheduling the instructions and processes in a lot of ways.
3. They are mostly liable for choosing the tasks to be presented into the method and choosing which instruction or process to run.

Types of CPU Schedulers

There are three types of CPU schedulers.

1. Long-term CPU Scheduler

Long-term type of scheduler is besides called as Job CPU Scheduler. It chooses an equalized mix of Input or Output compelled and Central Processing Unit bound instructions from the new state that is secondary memory. Then, it fills the chosen instructions into the ready state for the main memory for implementation and execution.

2. Short-term CPU Scheduler

THE short-term CPU scheduler is also called as CPU Scheduler. It settles that process to implement or execute subsequent from the wait or ready queue. once the short-term scheduler chooses the process, the contributor assigns the chose process to the Central Processing Unit for implementation and execution.

3. Medium-term Scheduler

Medium-term CPU scheduler switches-out the instructions from main type memory to secondary type memory to open the main kind of memory when needed. Thus, a medium-term CPU scheduler minimizes the amount of multitasking and multiprogramming. After some duration when the main space or memory turns to be available, medium-term CPU scheduler exchanges-in the changed-out instruction to the main space or memory and its implementation or execution is restarted from the situation it left off. exchanging may, in addition, be needed to enhance the process and instruction mix.

Criteria for Scheduling

CPU scheduling criteria are as follows:

1. CPU utilization: maintain and keep the Central Processing Unit as engaged as possible
    
2. Throughput: number of instructions or processes that absolute their implementation and execution per unit time.
3. Turnaround time: it is an amount of duration to implement or execute a specific process.
4. Waiting time: it is an amount of time in which a method has been expecting in the queue of ready process.
5. Response time: it is an amount of time that takes from a request process.