SHORT REVIEW ABOUT CPU SCHEDULING

INTRODUCTION

Modern operating system become more complex, when they switch from a single task to multitasking environment. CPU scheduling is an essential operating system task, therefore its scheduling is central to operating system design. When there is more than one process in the ready queue waiting in turn to be assigned to the CPU, the operating system must decide through the scheduler the order of execution in which they execute. Allocating CPU to a process requires careful attention to assure fairness and avoid process starvation for CPU.

CPU scheduling play a vital role by switching the CPU among several process. The aim of operating system to allow a number of processes concurrently in order to maximize the CPU utilization. In a multi-programmed Operating system, a process is executed until it must wait for the completion of some input-output request. Besides that, CPU scheduling determines which process run when there are multiple run-able processes. CPU scheduling is important because it can have a big effect on resource utilization and other performance parameters.

CPU SCHEDULING

CPU Scheduling is a process of determining which process will own CPU for execution while another process is on hold. The main task of CPU scheduling is to make sure that whenever the CPU remains idle, the OS at least select one of the processes available in the ready queue for execution. The selection process will be carried out by the CPU scheduler. It selects one of the processes in memory that are ready for execution.

TYPES OF CPU SCHEDULING

Here are two kinds of Scheduling methods:

Preemptive Scheduling

In Preemptive Scheduling, the tasks are mostly assigned with their priorities. Sometimes it is important to run a task with a higher priority before another lower priority task, even if the lower priority task is still running. The lower priority task holds for some time and resumes when the higher priority task finishes its execution.

Non-Preemptive Scheduling

In this type of scheduling method, the CPU has been allocated to a specific process. The process that keeps the CPU busy will release the CPU either by switching context or terminating. It is the only method that can be used for various hardware platforms. That's because it doesn't need special hardware (for example, a timer) like preemptive scheduling.

When scheduling is Preemptive or Non-Preemptive?

To determine if scheduling is preemptive or non-preemptive, consider these four parameters:

1. A process switches from the running to the waiting state.


2. Specific process switches from the running state to the ready state.


3. Specific process switches from the waiting state to the ready state.


4. Process finished its execution and terminated.

Only conditions 1 and 4 apply, the scheduling is called non- preemptive.

All other scheduling are preemptive.

CPU SCHEDULING CRITERIA

There are many different criteria to check when considering the "best" scheduling algorithm, they are:

CPU Utilization: To make out the best use of CPU and not to waste any CPU cycle, CPU would be working most of the time (Ideally 100% of the time). Considering a real system, CPU usage should range from 40% (lightly loaded) to 90% (heavily loaded.)

Throughput: It is the total number of processes completed per unit time or rather say total amount of work done in a unit of time. This may range from 10/second to 1/hour depending on the specific processes.

Turnaround Time: It is the amount of time taken to execute a particular process, i.e. The interval from time of submission of the process to the time of completion of the process (Wall clock time).

Waiting Time: The sum of the periods spent waiting in the ready queue amount of time a process has been waiting in the ready queue to acquire get control on the CPU.

Load Average: It is the average number of processes residing in the ready queue waiting for their turn to get into the CPU.

Response Time: Amount of time it takes from when a request was submitted until the first response is produced. Remember, it is the time till the first response and not the completion of process execution (final response).

In general CPU utilization and Throughput are maximized and other factors are reduced for proper optimization.

A CPU scheduling algorithm tries to maximize and minimize the following:



TYPES OF CPU SCHEDULING ALGORITHM

There are mainly six types of process scheduling algorithms

1. First Come First Serve (FCFS)


2. Shortest-Job-First (SJF) Scheduling


3. Shortest Remaining Time


4. Priority Scheduling


5. Round Robin Scheduling


6. Multilevel Queue Scheduling

First Come First Serve

First Come First Serve is the full form of FCFS. It is the easiest and most simple CPU scheduling algorithm. In this type of algorithm, the process which requests the CPU gets the CPU allocation first. This scheduling method can be managed with a FIFO queue.

As the process enters the ready queue, its PCB (Process Control Block) is linked with the tail of the queue. So, when CPU becomes free, it should be assigned to the process at the beginning of the queue.

Characteristics of FCFS method:

• It offers non-preemptive and pre-emptive scheduling algorithm.


• Jobs are always executed on a first-come, first-serve basis


• It is easy to implement and use.


• However, this method is poor in performance, and the general wait time is quite high.

Shortest Remaining Time

The full form of SRT is Shortest remaining time. It is also known as SJF preemptive scheduling. In this method, the process will be allocated to the task, which is closest to its completion. This method prevents a newer ready state process from holding the completion of an older process.

Characteristics of SRT scheduling method:

• This method is mostly applied in batch environments where short jobs are required to be given preference.


• This is not an ideal method to implement it in a shared system where the required CPU time is unknown.


• Associate with each process as the length of its next CPU burst. So that operating system uses these lengths, which helps to schedule the process with the shortest possible time.

Priority Based Scheduling

Priority scheduling is a method of scheduling processes based on priority. In this method, the scheduler selects the tasks to work as per the priority.

Priority scheduling also helps OS to involve priority assignments. The processes with higher priority should be carried out first, whereas jobs with equal priorities are carried out on a round-robin or FCFS basis. Priority can be decided based on memory requirements, time requirements, etc.

Round-Robin Scheduling

Round robin is the oldest, simplest scheduling algorithm. The name of this algorithm comes from the round-robin principle, where each person gets an equal share of something in turn. It is mostly used for scheduling algorithms in multitasking. This algorithm method helps for starvation free execution of processes.

Characteristics of Round-Robin Scheduling

• Round robin is a hybrid model which is clock-driven


• Time slice should be minimum, which is assigned for a specific task to be processed. However, it may vary for different processes.


• It is a real time system which responds to the event within a specific time limit.

Shortest Job First

SJF is a full form of (Shortest job first) is a scheduling algorithm in which the process with the shortest execution time should be selected for execution next. This scheduling method can be preemptive or non-preemptive. It significantly reduces the average waiting time for other processes awaiting execution.

Characteristics of SJF Scheduling

• It is associated with each job as a unit of time to complete.


• In this method, when the CPU is available, the next process or job with the shortest completion time will be executed first.


• It is implemented with non-preemptive policy.


• This algorithm method is useful for batch-type processing, where waiting for jobs to complete is not critical.


• It improves job output by offering shorter jobs, which should be executed first, which mostly have a shorter turnaround time.

Multiple-Level Queues Scheduling

This algorithm separates the ready queue into various separate queues. In this method, processes are assigned to a queue based on a specific property of the process, like the process priority, size of the memory, etc.

However, this is not an independent scheduling OS algorithm as it needs to use other types of algorithms in order to schedule the jobs.

Characteristic of Multiple-Level Queues Scheduling:

• Multiple queues should be maintained for processes with some characteristics.


• Every queue may have its separate scheduling algorithms.


• Priorities are given for each queue.