Operating System Scheduling algorithms

A Process Scheduler schedules different processes to be assigned to the CPU based on particular scheduling algorithms. There are six popular process scheduling algorithms which we are going to discuss in this chapter −

  • First-Come, First-Served (FCFS) Scheduling
  • Shortest-Job-Next (SJN) Scheduling
  • Priority Scheduling
  • Shortest Remaining Time
  • Round Robin(RR) Scheduling
  • Multiple-Level Queues Scheduling

These algorithms are either non-preemptive or preemptive. 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.

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.
1) 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.
2) 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.
3) 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.

4) 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.
5) 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.
6) 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.
The Purpose of a Scheduling algorithm

Here are the reasons for using a scheduling algorithm:

  • The CPU uses scheduling to improve its efficiency.
  • It helps you to allocate resources among competing processes.
  • The maximum utilization of CPU can be obtained with multi-programming.
  • The processes which are to be executed are in ready queue.
Presented by USAMA SALEEM

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