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14.2: Scheduling Algorithms

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Patrick McClanahan
San Joaquin Delta College

Scheduling Algorithms

Scheduling algorithms are used for distributing resources among parties which simultaneously and asynchronously request them. Scheduling disciplines are used in routers (to handle packet traffic) as well as in operating systems (to share CPU time among both threads and processes), disk drives (I/O scheduling), printers (print spooler), most embedded systems, etc.

The main purposes of scheduling algorithms are to minimize resource starvation and to ensure fairness amongst the parties utilizing the resources. Scheduling deals with the problem of deciding which of the outstanding requests is to be allocated resources. There are many different scheduling algorithms. In this section, we introduce several of them.

In packet-switched computer networks and other statistical multiplexing, the notion of a scheduling algorithm is used as an alternative to first-come first-served queuing of data packets.

The simplest best-effort scheduling algorithms are round-robin, fair queuing (a max-min fair scheduling algorithm), proportionally fair scheduling and maximum throughput. If differentiated or guaranteed quality of service is offered, as opposed to best-effort communication, weighted fair queuing may be utilized.

In advanced packet radio wireless networks such as HSDPA (High-Speed Downlink Packet Access) 3.5G cellular system, channel-dependent scheduling may be used to take advantage of channel state information. If the channel conditions are favourable, the throughput and system spectral efficiency may be increased. In even more advanced systems such as LTE, the scheduling is combined by channel-dependent packet-by-packet dynamic channel allocation, or by assigning OFDMA multi-carriers or other frequency-domain equalization components to the users that best can utilize them.

First come, first served

First in, first out (FIFO), also known as first come, first served (FCFS), is the simplest scheduling algorithm. FIFO simply queues processes in the order that they arrive in the ready queue. This is commonly used for a task queue, for example as illustrated in this section.

Since context switches only occur upon process termination, and no reorganization of the process queue is required, scheduling overhead is minimal.
Throughput can be low, because long processes can be holding the CPU, causing the short processes to wait for a long time (known as the convoy effect).
No starvation, because each process gets chance to be executed after a definite time.
Turnaround time, waiting time and response time depend on the order of their arrival and can be high for the same reasons above.
No prioritization occurs, thus this system has trouble meeting process deadlines.
The lack of prioritization means that as long as every process eventually completes, there is no starvation. In an environment where some processes might not complete, there can be starvation.
It is based on queuing.

Shortest remaining time first

Similar to shortest job first (SJF). With this strategy the scheduler arranges processes with the least estimated processing time remaining to be next in the queue. This requires advanced knowledge or estimations about the time required for a process to complete.

If a shorter process arrives during another process' execution, the currently running process is interrupted (known as preemption), dividing that process into two separate computing blocks. This creates excess overhead through additional context switching. The scheduler must also place each incoming process into a specific place in the queue, creating additional overhead.
This algorithm is designed for maximum throughput in most scenarios.
Waiting time and response time increase as the process's computational requirements increase. Since turnaround time is based on waiting time plus processing time, longer processes are significantly affected by this. Overall waiting time is smaller than FIFO, however since no process has to wait for the termination of the longest process.
No particular attention is given to deadlines, the programmer can only attempt to make processes with deadlines as short as possible.
Starvation is possible, especially in a busy system with many small processes being run.
To use this policy we should have at least two processes of different priority

Fixed priority pre-emptive scheduling

The operating system assigns a fixed priority rank to every process, and the scheduler arranges the processes in the ready queue in order of their priority. Lower-priority processes get interrupted by incoming higher-priority processes.

Overhead is not minimal, nor is it significant.
FPPS has no particular advantage in terms of throughput over FIFO scheduling.
If the number of rankings is limited, it can be characterized as a collection of FIFO queues, one for each priority ranking. Processes in lower-priority queues are selected only when all of the higher-priority queues are empty.
Waiting time and response time depend on the priority of the process. Higher-priority processes have smaller waiting and response times.
Deadlines can be met by giving processes with deadlines a higher priority.
Starvation of lower-priority processes is possible with large numbers of high-priority processes queuing for CPU time.

Round-robin scheduling

The scheduler assigns a fixed time unit per process, and cycles through them. If process completes within that time-slice it gets terminated otherwise it is rescheduled after giving a chance to all other processes.

RR scheduling involves extensive overhead, especially with a small time unit.
Balanced throughput between FCFS/ FIFO and SJF/SRTF, shorter jobs are completed faster than in FIFO and longer processes are completed faster than in SJF.
Good average response time, waiting time is dependent on number of processes, and not average process length.
Because of high waiting times, deadlines are rarely met in a pure RR system.
Starvation can never occur, since no priority is given. Order of time unit allocation is based upon process arrival time, similar to FIFO.
If Time-Slice is large it becomes FCFS /FIFO or if it is short then it becomes SJF/SRTF.

Multilevel queue scheduling

This is used for situations in which processes are easily divided into different groups. For example, a common division is made between foreground (interactive) processes and background (batch) processes. These two types of processes have different response-time requirements and so may have different scheduling needs. It is very useful for shared memory problems.

Work-conserving schedulers

A work-conserving scheduler is a scheduler that always tries to keep the scheduled resources busy, if there are submitted jobs ready to be scheduled. In contrast, a non-work conserving scheduler is a scheduler that, in some cases, may leave the scheduled resources idle despite the presence of jobs ready to be scheduled.

Choosing a scheduling algorithm

When designing an operating system, a programmer must consider which scheduling algorithm will perform best for the use the system is going to see. There is no universal "best" scheduling algorithm, and many operating systems use extended or combinations of the scheduling algorithms above.

For example, Windows NT/XP/Vista uses a multilevel feedback queue, a combination of fixed-priority preemptive scheduling, round-robin, and first in, first out algorithms. In this system, threads can dynamically increase or decrease in priority depending on if it has been serviced already, or if it has been waiting extensively. Every priority level is represented by its own queue, with round-robin scheduling among the high-priority threads and FIFO among the lower-priority ones. In this sense, response time is short for most threads, and short but critical system threads get completed very quickly. Since threads can only use one time unit of the round-robin in the highest-priority queue, starvation can be a problem for longer high-priority threads.

Adapted from: "Scheduling (computing)" by Multiple Contributors , Wikipedia is licensed under CC BY-SA 3.0

CS401: Operating Systems

Unit 4: cpu scheduling.

Central Process Unit (CPU) scheduling deals with having more processes/threads than processors to handles those tasks, meaning how the CPU determines which jobs it is going to handle in what order. A good understanding of how a CPU scheduling algorithm works is essential to understanding how an Operating System works; a good algorithm will optimally allocate resources, allowing an efficient execution of all running programs. A poor algorithm, however, could result in any number of issues, from process being "starved out" to inefficient executing, resulting in poor computer performance. In this unit, we will first discuss the CPU problem statement and the goals of a good scheduling algorithm. Then, we will move on to learning about types of CPU scheduling, such as preemptive and non-preemptive. Finally, we will conclude the module with a discussion on some of the more common algorithms found in UNIX-based Operating Systems.

Completing this unit should take you approximately 11 hours.

Upon successful completion of this unit, you will be able to:

discuss CPU scheduling and its relevance to operating systems;
explain the general goals of CPU scheduling;
describe the differences between preemptive and non-preemptive scheduling; and
discuss four CPU scheduling algorithms.

4.1: Scheduling General Objective

Watch the first lecture from 54:30 to the end, and watch the second lecture until 31:00.

Read this document.

Read these notes.

Read the first seven slides.

4.4: Algorithms

Read these slides.

Unit 4 Exercises and Assessment

Complete both parts of the CPU scheduling simulations. You will need to have Java Installed on your computer. Follow all of the instructions provided. Your results should match the answers provided in the answer keys for Part 1 and Part 2 .

Review the material on CPU scheduling algorithms. Complete both problems. It might be best to draw out Gantt charts to represent the processes.

This assessment does not count towards your grade . It is just for practice!
You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
You can take this assessment as many times as you want, whenever you want.

Multiprocessor and Distributed Real-Time Scheduling

First Online: 24 July 2019

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K. Erciyes 8

Part of the book series: Computer Communications and Networks ((CCN))

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Scheduling of real-time tasks on multiprocessors and distributed hardware is a more difficult problem than the uniprocessor case. Finding an optimal schedule for given real-time tasks on a multiprocessor or a distributed system is an NP-hard problem and hence, heuristic algorithms are commonly employed to find suboptimal solutions. Partitioned scheduling used in multiprocessor systems involve assigning tasks to the processors using a suitable algorithm then scheduling the assigned tasks in each processor, commonly using a known single processor algorithm. Global scheduling algorithms schedule tasks to processors using a single ready queue. We review these algorithms and then look at distributed scheduling and load balancing in distributed (real-time) systems.

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Erciyes, K. (2019). Multiprocessor and Distributed Real-Time Scheduling. In: Distributed Real-Time Systems. Computer Communications and Networks. Springer, Cham. https://doi.org/10.1007/978-3-030-22570-4_9

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CPU Scheduling Algorithms in Operating Systems

What is 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:

A process switches from the running to the waiting state.
Specific process switches from the running state to the ready state.
Specific process switches from the waiting state to the ready state.
Process finished its execution and terminated.

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

All other scheduling are preemptive.

Important CPU scheduling Terminologies

Burst Time/Execution Time: It is a time required by the process to complete execution. It is also called running time.
Arrival Time: when a process enters in a ready state
Finish Time: when process complete and exit from a system
Multiprogramming: A number of programs which can be present in memory at the same time.
Jobs: It is a type of program without any kind of user interaction.
User: It is a kind of program having user interaction.
Process: It is the reference that is used for both job and user.
CPU/IO burst cycle: Characterizes process execution, which alternates between CPU and I/O activity. CPU times are usually shorter than the time of I/O.

CPU Scheduling Criteria

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

CPU utilization: CPU utilization is the main task in which the operating system needs to make sure that CPU remains as busy as possible. It can range from 0 to 100 percent. However, for the RTOS, it can be range from 40 percent for low-level and 90 percent for the high-level system.

Throughput: The number of processes that finish their execution per unit time is known Throughput. So, when the CPU is busy executing the process, at that time, work is being done, and the work completed per unit time is called Throughput.

Waiting time: Waiting time is an amount that specific process needs to wait in the ready queue.

Response time: It is an amount to time in which the request was submitted until the first response is produced.

Turnaround Time: Turnaround time is an amount of time to execute a specific process. It is the calculation of the total time spent waiting to get into the memory, waiting in the queue and, executing on the CPU. The period between the time of process submission to the completion time is the turnaround time.

Interval Timer

Timer interruption is a method that is closely related to preemption. When a certain process gets the CPU allocation, a timer may be set to a specified interval. Both timer interruption and preemption force a process to return the CPU before its CPU burst is complete.

Most of the multi-programmed operating system uses some form of a timer to prevent a process from tying up the system forever.

What is Dispatcher?

It is a module that provides control of the CPU to the process. The Dispatcher should be fast so that it can run on every context switch. Dispatch latency is the amount of time needed by the CPU scheduler to stop one process and start another.

Functions performed by Dispatcher:

Context Switching
Switching to user mode
Moving to the correct location in the newly loaded program.

Types of CPU scheduling Algorithm

There are mainly six types of process scheduling algorithms

First Come First Serve (FCFS)
Shortest-Job-First (SJF) Scheduling

Shortest Remaining Time

Priority Scheduling
Round Robin Scheduling
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.

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.

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.
CPU scheduling is a process of determining which process will own CPU for execution while another process is on hold.
In Preemptive Scheduling, the tasks are mostly assigned with their priorities.
In the Non-preemptive scheduling method, the CPU has been allocated to a specific process.
The burst time is the time required for the process to complete execution. It is also called running time.
CPU utilization is the main task in which the operating system needs to ensure that the CPU remains as busy as possible.
The number of processes that finish their execution per unit time is known Throughput.
Waiting time is an amount that a specific process needs to wait in the ready queue.
It is the amount of time in which the request was submitted until the first response is produced.
Turnaround time is the amount of time to execute a specific process.
Timer interruption is a method that is closely related to preemption.
A dispatcher is a module that provides control of the CPU to the process.
Six types of process scheduling algorithms are: 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.
In the First Come First Serve method , the process which requests the CPU gets the CPU allocation first.
In the Shortest Remaining time, the process will be allocated to the task closest to its completion.
In Priority Scheduling, the scheduler selects the tasks to work as per the priority.
Round robin scheduling works on the principle where each person gets an equal share of something in turn.
In the Shortest job first, the shortest execution time should be selected for execution next.
The multilevel scheduling method separates the ready queue into various separate queues. In this method, processes are assigned to a queue based on a specific property.
Round Robin Scheduling Algorithm with Example
Process Synchronization: Critical Section Problem in OS
Process Scheduling in OS: Long, Medium, Short Term Scheduler
Priority Scheduling Algorithm: Preemptive, Non-Preemptive EXAMPLE
Memory Management in OS: Contiguous, Swapping, Fragmentation
Shortest Job First (SJF): Preemptive, Non-Preemptive Example
Virtual Memory in OS: What is, Demand Paging, Advantages
Difference Between SSD and HDD

Java for Android
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Operating System Tutorial
What is an Operating System?
Functions of Operating System
Types of Operating Systems
Need and Functions of Operating Systems
Commonly Used Operating System

Structure of Operating System

Operating System Services
Introduction of System Call
System Programs in Operating System
Operating Systems Structures
History of Operating System
Booting and Dual Booting of Operating System

Types of OS

Batch Processing Operating System
Multiprogramming in Operating System
Time Sharing Operating System
What is a Network Operating System?
Real Time Operating System (RTOS)

Process Management

Introduction of Process Management
Process Table and Process Control Block (PCB)
Operations on Processes
Process Schedulers in Operating System
Inter Process Communication (IPC)
Context Switching in Operating System
Preemptive and Non-Preemptive Scheduling

CPU Scheduling in OS

Cpu scheduling in operating systems.

CPU Scheduling Criteria
Multiple-Processor Scheduling in Operating System
Thread Scheduling

Threads in OS

Thread in Operating System
Threads and its types in Operating System
Multithreading in Operating System

Process Synchronization

Introduction of Process Synchronization
Race Condition Vulnerability
Critical Section in Synchronization
Mutual Exclusion in Synchronization

Critical Section Problem Solution

Peterson's Algorithm in Process Synchronization
Semaphores in Process Synchronization
Semaphores and its types
Producer Consumer Problem using Semaphores | Set 1
Readers-Writers Problem | Set 1 (Introduction and Readers Preference Solution)
Dining Philosopher Problem Using Semaphores
Hardware Synchronization Algorithms : Unlock and Lock, Test and Set, Swap

Deadlocks & Deadlock Handling Methods

Introduction of Deadlock in Operating System
Conditions for Deadlock in Operating System
Banker's Algorithm in Operating System
Wait For Graph Deadlock Detection in Distributed System
Handling Deadlocks
Deadlock Prevention And Avoidance
Deadlock Detection And Recovery
Deadlock Ignorance in Operating System
Recovery from Deadlock in Operating System

Memory Management

Memory Management in Operating System
Implementation of Contiguous Memory Management Techniques
Non-Contiguous Allocation in Operating System
Compaction in Operating System
Best-Fit Allocation in Operating System
Worst-Fit Allocation in Operating Systems
First-Fit Allocation in Operating Systems
Fixed (or static) Partitioning in Operating System
Variable (or Dynamic) Partitioning in Operating System
Paging in Operating System
Segmentation in Operating System
Virtual Memory in Operating System

Page Replacement Algorithms

Page Replacement Algorithms in Operating Systems
Program for Page Replacement Algorithms | Set 2 (FIFO)
Belady's Anomaly in Page Replacement Algorithms
Optimal Page Replacement Algorithm
Program for Least Recently Used (LRU) Page Replacement algorithm
Techniques to handle Thrashing
Storage Management
File Systems in Operating System
File Allocation Methods
Free space management in Operating System
Disk Scheduling Algorithms
RAID (Redundant Arrays of Independent Disks)

OS Interview Questions

Last Minute Notes – Operating Systems
Operating System Interview Questions

OS Quiz and GATE PYQ's

OS Process Management
OS Memory Management
OS Input Output Systems
OS CPU Scheduling
50 Operating System MCQs with Answers

Scheduling of processes/work is done to finish the work on time. CPU Scheduling is a process that allows one process to use the CPU while another process is delayed (in standby) due to unavailability of any resources such as I / O etc, thus making full use of the CPU. The purpose of CPU Scheduling is to make the system more efficient, faster, and fairer.

Tutorial on CPU Scheduling Algorithms in Operating System

Whenever the CPU becomes idle, the operating system must select one of the processes in the line ready for launch. The selection process is done by a temporary (CPU) scheduler. The Scheduler selects between memory processes ready to launch and assigns the CPU to one of them.

Table of Contents

What is a Process?
How is Process Memory used for efficient operation?

What is Process Scheduling?

Why do we need to schedule processes.

Objectives of Process Scheduling Algorithm
What are the different terminologies?
Things to take care while designing CPU Scheduling Algorithm
Characteristics of FCFS
Advantages of FCFS
Disadvantages of FCFS
Characteristics of SJF
Advantages of SJF
Disadvantages of SJF
Characteristics of LJF
Advantages of LJF
Disadvantages of LJF
Characteristics of Priority Scheduling
Advantages of Priority Scheduling
Disadvantages of Priority Scheduling
Characteristics of Round Robin
Advantages of Round Robin
Characteristics of SRTF
Advantages of SRTF
Disadvantages of SRTF
Characteristics of LRTF
Advanatges of LRTF
Disadvantages of LRTF
Characteristics of HRRN
Advantages of HRRN
Disadvantages of HRRN
Advantages of multilevel queue scheduling
Disadvantages of multilevel queue scheduling
Characteristics of MLFQ
Advantages of MLFQ
Disadvantages of MLFQ

What is a process?

In computing, a process is the instance of a computer program that is being executed by one or many threads . It contains the program code and its activity. Depending on the operating system (OS), a process may be made up of multiple threads of execution that execute instructions concurrently.

How is process memory used for efficient operation?

The process memory is divided into four sections for efficient operation:

The text category is composed of integrated program code, which is read from fixed storage when the program is launched.
The data class is made up of global and static variables, distributed and executed before the main action.
Heap is used for flexible, or dynamic memory allocation and is managed by calls to new, delete, malloc, free, etc.
The stack is used for local variables. The space in the stack is reserved for local variables when it is announced.

To know further, you can refer to our detailed article on States of a Process in Operating system .

Process Scheduling is the process of the process manager handling the removal of an active process from the CPU and selecting another process based on a specific strategy.

Process Scheduling is an integral part of Multi-programming applications. Such operating systems allow more than one process to be loaded into usable memory at a time and the loaded shared CPU process uses repetition time.

There are three types of process schedulers :

Long term or Job Scheduler
Short term or CPU Scheduler
Medium-term Scheduler
Scheduling is important in many different computer environments. One of the most important areas is scheduling which programs will work on the CPU. This task is handled by the Operating System (OS) of the computer and there are many different ways in which we can choose to configure programs.
Process Scheduling allows the OS to allocate CPU time for each process. Another important reason to use a process scheduling system is that it keeps the CPU busy at all times. This allows you to get less response time for programs.
Considering that there may be hundreds of programs that need to work, the OS must launch the program, stop it, switch to another program, etc. The way the OS configures the system to run another in the CPU is called “ context switching ”. If the OS keeps context-switching programs in and out of the provided CPUs, it can give the user a tricky idea that he or she can run any programs he or she wants to run, all at once.
So now that we know we can run 1 program at a given CPU, and we know we can change the operating system and remove another one using the context switch, how do we choose which programs we need. run, and with what program?
That’s where scheduling comes in! First, you determine the metrics, saying something like “the amount of time until the end”. We will define this metric as “the time interval between which a function enters the system until it is completed”. Second, you decide on a metrics that reduces metrics. We want our tasks to end as soon as possible.

What is the need for CPU scheduling algorithm?

CPU scheduling is the process of deciding which process will own the CPU to use while another process is suspended. The main function of the CPU scheduling is to ensure that whenever the CPU remains idle, the OS has at least selected one of the processes available in the ready-to-use line.

In Multiprogramming , if the long-term scheduler selects multiple I / O binding processes then most of the time, the CPU remains an idle. The function of an effective program is to improve resource utilization.

If most operating systems change their status from performance to waiting then there may always be a chance of failure in the system. So in order to minimize this excess, the OS needs to schedule tasks in order to make full use of the CPU and avoid the possibility of deadlock.

Objectives of Process Scheduling Algorithm:

Utilization of CPU at maximum level. Keep CPU as busy as possible .
Allocation of CPU should be fair .
Throughput should be Maximum . i.e. Number of processes that complete their execution per time unit should be maximized.
Minimum turnaround time , i.e. time taken by a process to finish execution should be the least.
There should be a minimum waiting time and the process should not starve in the ready queue.
Minimum response time. It means that the time when a process produces the first response should be as less as possible.

What are the different terminologies to take care of in any CPU Scheduling algorithm?

Arrival Time: Time at which the process arrives in the ready queue.
Completion Time: Time at which process completes its execution.
Burst Time: Time required by a process for CPU execution.
Turn Around Time: Time Difference between completion time and arrival time.

Turn Around Time = Completion Time – Arrival Time

Waiting Time(W.T): Time Difference between turn around time and burst time.

Waiting Time = Turn Around Time – Burst Time

Things to take care while designing a CPU Scheduling algorithm?

Different CPU Scheduling algorithms have different structures and the choice of a particular algorithm depends on a variety of factors. Many conditions have been raised to compare CPU scheduling algorithms.

The criteria include the following:

CPU utilization: The main purpose of any CPU algorithm is to keep the CPU as busy as possible. Theoretically, CPU usage can range from 0 to 100 but in a real-time system, it varies from 40 to 90 percent depending on the system load.
Throughput: The average CPU performance is the number of processes performed and completed during each unit. This is called throughput. The output may vary depending on the length or duration of the processes.
Turn round Time: For a particular process, the important conditions are how long it takes to perform that process. The time elapsed from the time of process delivery to the time of completion is known as the conversion time. Conversion time is the amount of time spent waiting for memory access, waiting in line, using CPU, and waiting for I / O.
Waiting Time: The Scheduling algorithm does not affect the time required to complete the process once it has started performing. It only affects the waiting time of the process i.e. the time spent in the waiting process in the ready queue.
Response Time: In a collaborative system, turn around time is not the best option. The process may produce something early and continue to computing the new results while the previous results are released to the user. Therefore another method is the time taken in the submission of the application process until the first response is issued. This measure is called response time.

What are the different types of CPU Scheduling Algorithms?

There are mainly two types of scheduling methods:

Preemptive Scheduling : Preemptive scheduling is used when a process switches from running state to ready state or from the waiting state to the ready state.
Non-Preemptive Scheduling : Non-Preemptive scheduling is used when a process terminates , or when a process switches from running state to waiting state.

Different types of CPU Scheduling Algorithms

Let us now learn about these CPU scheduling algorithms in operating systems one by one:

1. First Come First Serve:

FCFS considered to be the simplest of all operating system scheduling algorithms. First come first serve scheduling algorithm states that the process that requests the CPU first is allocated the CPU first and is implemented by using FIFO queue .

Characteristics of FCFS:

FCFS supports non-preemptive and preemptive CPU scheduling algorithms.
Tasks are always executed on a First-come, First-serve concept.
FCFS is easy to implement and use.
This algorithm is not much efficient in performance, and the wait time is quite high.

Advantages of FCFS:

Easy to implement
First come, first serve method

Disadvantages of FCFS:

FCFS suffers from Convoy effect .
The average waiting time is much higher than the other algorithms.
FCFS is very simple and easy to implement and hence not much efficient.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on First come, First serve Scheduling.

2. Shortest Job First(SJF):

Shortest job first (SJF) is a scheduling process that selects the waiting process with the smallest execution time to execute next. This scheduling method may or may not be preemptive. Significantly reduces the average waiting time for other processes waiting to be executed. The full form of SJF is Shortest Job First.

Characteristics of SJF:

Shortest Job first has the advantage of having a minimum average waiting time among all operating system scheduling algorithms.
It is associated with each task as a unit of time to complete.
It may cause starvation if shorter processes keep coming. This problem can be solved using the concept of ageing.

Advantages of Shortest Job first:

As SJF reduces the average waiting time thus, it is better than the first come first serve scheduling algorithm.
SJF is generally used for long term scheduling

Disadvantages of SJF:

One of the demerit SJF has is starvation.
Many times it becomes complicated to predict the length of the upcoming CPU request

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on Shortest Job First.

3. Longest Job First(LJF):

Longest Job First(LJF) scheduling process is just opposite of shortest job first (SJF), as the name suggests this algorithm is based upon the fact that the process with the largest burst time is processed first. Longest Job First is non-preemptive in nature.

Characteristics of LJF:

Among all the processes waiting in a waiting queue, CPU is always assigned to the process having largest burst time.
If two processes have the same burst time then the tie is broken using FCFS i.e. the process that arrived first is processed first.
LJF CPU Scheduling can be of both preemptive and non-preemptive types.

Advantages of LJF:

No other task can schedule until the longest job or process executes completely.
All the jobs or processes finish at the same time approximately.

Disadvantages of LJF:

Generally, the LJF algorithm gives a very high average waiting time and average turn-around time for a given set of processes.
This may lead to convoy effect.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on the Longest job first scheduling .

4. Priority Scheduling:

Preemptive Priority CPU Scheduling Algorithm is a pre-emptive method of CPU scheduling algorithm that works based on the priority of a process. In this algorithm, the editor sets the functions to be as important, meaning that the most important process must be done first. In the case of any conflict, that is, where there is more than one process with equal value, then the most important CPU planning algorithm works on the basis of the FCFS (First Come First Serve) algorithm.

Characteristics of Priority Scheduling:

Schedules tasks based on priority.
When the higher priority work arrives and a task with less priority is executing, the higher priority proess will takes the place of the less priority proess and
The later is suspended until the execution is complete.
Lower is the number assigned, higher is the priority level of a process.

Advantages of Priority Scheduling:

The average waiting time is less than FCFS
Less complex

Disadvantages of Priority Scheduling:

One of the most common demerits of the Preemptive priority CPU scheduling algorithm is the Starvation Problem . This is the problem in which a process has to wait for a longer amount of time to get scheduled into the CPU. This condition is called the starvation problem.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on Priority Preemptive Scheduling algorithm .

5. Round robin:

Round Robin is a CPU scheduling algorithm where each process is cyclically assigned a fixed time slot. It is the preemptive version of First come First Serve CPU Scheduling algorithm . Round Robin CPU Algorithm generally focuses on Time Sharing technique.

Characteristics of Round robin:

It’s simple, easy to use, and starvation-free as all processes get the balanced CPU allocation.
One of the most widely used methods in CPU scheduling as a core.
It is considered preemptive as the processes are given to the CPU for a very limited time.

Advantages of Round robin:

Round robin seems to be fair as every process gets an equal share of CPU.
The newly created process is added to the end of the ready queue.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on the Round robin Scheduling algorithm .

6. Shortest Remaining Time First:

Shortest remaining time first is the preemptive version of the Shortest job first which we have discussed earlier where the processor is allocated to the job closest to completion. In SRTF the process with the smallest amount of time remaining until completion is selected to execute.

Characteristics of Shortest remaining time first:

SRTF algorithm makes the processing of the jobs faster than SJF algorithm, given it’s overhead charges are not counted.
The context switch is done a lot more times in SRTF than in SJF and consumes the CPU’s valuable time for processing. This adds up to its processing time and diminishes its advantage of fast processing.

Advantages of SRTF:

In SRTF the short processes are handled very fast.
The system also requires very little overhead since it only makes a decision when a process completes or a new process is added.

Disadvantages of SRTF:

Like the shortest job first, it also has the potential for process starvation.
Long processes may be held off indefinitely if short processes are continually added.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on the shortest remaining time first .

7. Longest Remaining Time First:

The longest remaining time first is a preemptive version of the longest job first scheduling algorithm. This scheduling algorithm is used by the operating system to program incoming processes for use in a systematic way. This algorithm schedules those processes first which have the longest processing time remaining for completion.

Characteristics of longest remaining time first:

Among all the processes waiting in a waiting queue, the CPU is always assigned to the process having the largest burst time.
LRTF CPU Scheduling can be of both preemptive and non-preemptive.
No other process can execute until the longest task executes completely.

Disadvantages of LRTF:

This algorithm gives a very high average waiting time and average turn-around time for a given set of processes.
This may lead to a convoy effect.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on the longest remaining time first .

8. Highest Response Ratio Next:

Highest Response Ratio Next is a non-preemptive CPU Scheduling algorithm and it is considered as one of the most optimal scheduling algorithms. The name itself states that we need to find the response ratio of all available processes and select the one with the highest Response Ratio. A process once selected will run till completion.

Characteristics of Highest Response Ratio Next:

The criteria for HRRN is Response Ratio, and the mode is Non-Preemptive.
HRRN is considered as the modification of Shortest Job First to reduce the problem of starvation .
In comparison with SJF, during the HRRN scheduling algorithm, the CPU is allotted to the next process which has the highest response ratio and not to the process having less burst time.

Response Ratio = (W + S)/S Here, W is the waiting time of the process so far and S is the Burst time of the process.

Advantages of HRRN:

HRRN Scheduling algorithm generally gives better performance than the shortest job first Scheduling.
There is a reduction in waiting time for longer jobs and also it encourages shorter jobs.

Disadvantages of HRRN:

The implementation of HRRN scheduling is not possible as it is not possible to know the burst time of every job in advance.
In this scheduling, there may occur an overload on the CPU.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on Highest Response Ratio Next .

9. Multiple Queue Scheduling:

Processes in the ready queue can be divided into different classes where each class has its own scheduling needs. For example, a common division is a foreground (interactive) process and a background (batch) process. These two classes have different scheduling needs. For this kind of situation Multilevel Queue Scheduling is used.

The description of the processes in the above diagram is as follows:

System Processes: The CPU itself has its process to run, generally termed as System Process.
Interactive Processes: An Interactive Process is a type of process in which there should be the same type of interaction.
Batch Processes: Batch processing is generally a technique in the Operating system that collects the programs and data together in the form of a batch before the processing starts.

Advantages of multilevel queue scheduling:

The main merit of the multilevel queue is that it has a low scheduling overhead.

Disadvantages of multilevel queue scheduling:

Starvation problem
It is inflexible in nature

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on Multilevel Queue Scheduling .

10. Multilevel Feedback Queue Scheduling: :

Multilevel Feedback Queue Scheduling (MLFQ) CPU Scheduling is like Multilevel Queue Scheduling but in this process can move between the queues. And thus, much more efficient than multilevel queue scheduling.

Characteristics of Multilevel Feedback Queue Scheduling:

In a multilevel queue-scheduling algorithm, processes are permanently assigned to a queue on entry to the system, and processes are not allowed to move between queues.
As the processes are permanently assigned to the queue, this setup has the advantage of low scheduling overhead,
But on the other hand disadvantage of being inflexible.

Advantages of Multilevel feedback queue scheduling:

It is more flexible
It allows different processes to move between different queues

Disadvantages of Multilevel feedback queue scheduling:

It also produces CPU overheads
It is the most complex algorithm.

To learn about how to implement this CPU scheduling algorithm, please refer to our detailed article on Multilevel Feedback Queue Scheduling .

Comparison between various CPU Scheduling algorithms

Here is a brief comparison between different CPU scheduling algorithms:

Consider a system which requires 40-time units of burst time. The Multilevel feedback queue scheduling is used and time quantum is 2 unit for the top queue and is incremented by 5 unit at each level, then in what queue the process will terminate the execution?
Which of the following is false about SJF? S1: It causes minimum average waiting time S2: It can cause starvation (A) Only S1 (B) Only S2 (C) Both S1 and S2 (D) Neither S1 nor S2 Answer (D) S1 is true SJF will always give minimum average waiting time. S2 is true SJF can cause starvation.
Consider the following table of arrival time and burst time for three processes P0, P1 and P2. (GATE-CS-2011)
The pre-emptive shortest job first scheduling algorithm is used. Scheduling is carried out only at arrival or completion of processes. What is the average waiting time for the three processes? (A) 5.0 ms (B) 4.33 ms (C) 6.33 (D) 7.33 Solution : Answer: – (A) Process P0 is allocated processor at 0 ms as there is no other process in the ready queue. P0 is preempted after 1 ms as P1 arrives at 1 ms and burst time for P1 is less than remaining time of P0. P1 runs for 4ms. P2 arrived at 2 ms but P1 continued as burst time of P2 is longer than P1. After P1 completes, P0 is scheduled again as the remaining time for P0 is less than the burst time of P2. P0 waits for 4 ms, P1 waits for 0 ms and P2 waits for 11 ms. So average waiting time is (0+4+11)/3 = 5.
Consider the following set of processes, with the arrival times and the CPU-burst times given in milliseconds (GATE-CS-2004)
What is the average turnaround time for these processes with the preemptive shortest remaining processing time first (SRPT) algorithm ? (A) 5.50 (B) 5.75 (C) 6.00 (D) 6.25 Answer (A) Solution: The following is Gantt Chart of execution
Turn Around Time = Completion Time – Arrival Time Avg Turn Around Time = (12 + 3 + 6+ 1)/4 = 5.50
An operating system uses the Shortest Remaining Time First (SRTF) process scheduling algorithm. Consider the arrival times and execution times for the following processes:
What is the total waiting time for process P2? (A) 5 (B) 15 (C) 40 (D) 55 Answer (B) At time 0, P1 is the only process, P1 runs for 15 time units. At time 15, P2 arrives, but P1 has the shortest remaining time. So P1 continues for 5 more time units. At time 20, P2 is the only process. So it runs for 10 time units At time 30, P3 is the shortest remaining time process. So it runs for 10 time units At time 40, P2 runs as it is the only process. P2 runs for 5 time units. At time 45, P3 arrives, but P2 has the shortest remaining time. So P2 continues for 10 more time units. P2 completes its execution at time 55

Total waiting time for P2 = Completion time – (Arrival time + Execution time) = 55 – (15 + 25) = 15

Quiz on CPU Scheduling
CPU Scheduling in operating system
Comparison of Different CPU Scheduling Algorithms in OS

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What Is CPU Scheduling

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Introduction

When it comes to managing a computer’s resources efficiently, CPU scheduling plays a vital role. In the world of operating systems, CPU scheduling is the process of determining which tasks or processes should be executed by the CPU at a given time. As the CPU is the heart of a computer system, effective CPU scheduling algorithms are crucial for optimizing performance, improving responsiveness, and ensuring fairness in sharing resources.

CPU scheduling becomes even more critical in multi-user systems, where multiple processes are vying for CPU time. The goal of CPU scheduling algorithms is to minimize the average response time, maximize throughput, and maintain fairness among processes.

Understanding the intricacies of CPU scheduling is important for system administrators, software developers, and computer science enthusiasts alike. It provides insights into how computer systems manage and allocate resources to deliver a smooth and efficient computing experience.

In this article, we will delve into the concept of CPU scheduling, explore its importance, and discuss various CPU scheduling algorithms used in modern operating systems. By the end of this article, you will have a comprehensive understanding of CPU scheduling and its significance in optimizing system performance.

What is CPU Scheduling?

CPU scheduling is a fundamental component of modern operating systems and is the mechanism that determines which tasks or processes get access to the CPU’s processing time. In simpler terms, it is the algorithmic strategy used to schedule the execution of processes in a computer system.

When a computer system has multiple processes competing for CPU resources, it becomes crucial to allocate CPU time efficiently to maximize overall system performance. CPU scheduling algorithms consider factors like process priority, burst time, waiting time, and resource dependencies to make informed decisions on which process should be executed next.

At its core, CPU scheduling aims to achieve several objectives, including:

Fairness: Ensuring that each process gets a fair share of the CPU’s processing time.
Efficiency: Maximizing CPU utilization and minimizing idle time.
Responsiveness: Providing quick response times to user requests and interactive tasks.
Throughput: Maximizing the number of processes completed within a given time frame.

It’s important to note that different CPU scheduling algorithms can be employed based on the specific needs and characteristics of the system. The choice of algorithm can significantly impact system performance and responsiveness.

Nowadays, multi-core processors are common, meaning there are multiple CPUs available for executing processes concurrently. On these systems, the task of CPU scheduling extends to distributing processes across multiple cores efficiently.

In the next section, we will explore why CPU scheduling is of utmost importance in operating systems and how it contributes to the overall system performance and user experience.

Why is CPU Scheduling important?

CPU scheduling is a critical aspect of operating systems as it directly impacts system performance, responsiveness, and resource utilization. Let’s delve into why CPU scheduling is so important:

1. Optimizing CPU Utilization: Efficient CPU scheduling ensures that the CPU remains busy, executing processes as much as possible. By minimizing idle time and maximizing CPU utilization, CPU scheduling allows the system to handle more tasks and achieve higher throughput.

2. Improving Responsiveness: In interactive systems, such as desktops or servers that handle user requests, responsiveness is key. By employing appropriate CPU scheduling algorithms, the operating system can provide quick response times to user actions, making the system feel more snappy and responsive.

3. Enhancing System Performance: CPU scheduling algorithms play a crucial role in optimizing system performance. By making intelligent decisions on task priorities and execution order, CPU scheduling can minimize the average response time, reduce process waiting time, and deliver faster task completion.

4. Enabling Fair Resource Allocation: CPU scheduling ensures fairness by allocating CPU time fairly among competing processes. It prevents any single process from monopolizing the CPU for an extended period, thus creating a balanced resource distribution among running processes.

5. Supporting Real-Time Systems: In real-time systems, where strict timing requirements must be met, such as in aviation or industrial control systems, CPU scheduling plays a vital role in meeting deadlines. Real-time scheduling algorithms ensure that critical tasks are executed on time, guaranteeing system reliability.

6. Minimizing Bottlenecks: CPU scheduling helps in identifying and resolving potential bottlenecks within the system. By analyzing the performance of different processes and their resource requirements, CPU scheduling algorithms can allocate resources more effectively, preventing resource contention and enhancing overall system performance.

Overall, CPU scheduling is a crucial component of any operating system, and its effective implementation is essential for achieving optimal system performance, responsiveness, and resource utilization.

Different CPU Scheduling Algorithms

Several CPU scheduling algorithms have been developed over the years, each with its unique characteristics and suitability for specific scenarios. Let’s explore some of the commonly used CPU scheduling algorithms:

1. First-Come, First-Served (FCFS) Scheduling: This is a non-preemptive scheduling algorithm where processes are executed in the order they arrive. The CPU is allocated to the first process in the queue, and it continues execution until completion or when it voluntarily relinquishes control. FCFS provides fairness but may suffer from poor responsiveness when long-running processes block others from executing.

2. Shortest Job Next (SJN) Scheduling: Also known as Shortest Job First (SJF), this algorithm selects the process with the shortest burst time to execute first. SJN can minimize average waiting time and turnaround time, but it requires knowledge of the burst time beforehand, which is often unrealistic in practice.

3. Round Robin (RR) Scheduling: In this algorithm, processes are executed in a cyclic fashion, with each process receiving a fixed time slice or quantum of CPU time. If a process does not complete execution within the time slice, it is preempted and moved to the back of the queue. RR provides fairness and responsiveness, but can suffer from high context switch overhead and may not be suitable for long-running tasks.

4. Priority Scheduling: Each process is assigned a priority value, and the CPU is allocated to the process with the highest priority at any given time. Priority can be static or dynamic, with static priority assigned based on process characteristics and dynamic priority adjusted during runtime. Priority scheduling is flexible but requires careful management to prevent starvation and ensure fair resource allocation.

5. Multilevel Queue Scheduling: This algorithm divides processes into multiple queues based on priority or other attributes. Each queue has its own scheduling algorithm, allowing different classes of processes to be managed separately. For example, interactive tasks could be assigned to a high-priority queue for quick response, while background tasks are placed in a lower-priority queue.

These are just a few examples of CPU scheduling algorithms, and there are many variations and hybrid approaches that have been developed to address different system requirements and constraints.

CPU scheduling algorithms are a crucial aspect of designing and implementing efficient operating systems. The choice of algorithm depends on factors such as system workload, task characteristics, and performance goals. By understanding these different algorithms, system administrators and developers can make informed decisions to enhance system performance and responsiveness.

First-Come, First-Served (FCFS) Scheduling

First-Come, First-Served (FCFS) Scheduling is one of the simplest and most straightforward CPU scheduling algorithms. In FCFS, the processes that arrive first are executed first. The CPU is allocated to the first process in the ready queue, and it continues executing until completion or when it voluntarily relinquishes control, such as when it requests an I/O operation.

FCFS is a non-preemptive scheduling algorithm, which means that once a process starts executing, it holds the CPU until it completes or requests an I/O operation. Only then will the CPU be allocated to the next process in the queue.

While FCFS provides fairness in terms of executing processes in the order they arrive, it may suffer from poor responsiveness, especially if there are long-running processes that block others from executing. This is known as the “convoy effect,” where a single long process causes other shorter processes to wait, leading to increased waiting time and reduced system performance.

One advantage of FCFS is its simplicity in implementation. Its non-preemptive nature avoids issues like race conditions and synchronization problems that can occur in preemptive scheduling algorithms. Additionally, FCFS does not require knowledge of burst times in advance, making it easier to use in practice.

However, FCFS has limitations, and its performance may not be optimal in all scenarios. For example, if a CPU-bound process arrives first and continues to execute for a long time, it could significantly delay the execution of other processes, especially interactive or time-sensitive tasks.

In summary, FCFS is a simple and fair CPU scheduling algorithm. While it provides straightforward implementation and avoids certain complexities of preemptive algorithms, it may suffer from poor responsiveness and delay other tasks. As such, understanding the characteristics and trade-offs of FCFS is important for designing efficient CPU scheduling strategies in operating systems.

Shortest Job Next (SJN) Scheduling

Shortest Job Next (SJN) Scheduling, also known as Shortest Job First (SJF) Scheduling, is a CPU scheduling algorithm that selects the process with the shortest burst time to be executed next. The idea behind SJN is to minimize the average waiting time and turnaround time of processes.

In SJN scheduling, the CPU is allocated to the process that has the smallest total execution time or burst time. The assumption is that shorter processes tend to complete more quickly, resulting in faster turnaround times and improved system performance.

One challenge in implementing SJN is the difficulty of accurately predicting the burst time of each process before it starts execution. In practical scenarios, burst times are often unknown, making it challenging to determine which process is actually the shortest. Therefore, in most cases, an estimate or approximation of burst time is used to make scheduling decisions.

SJN can be implemented in both preemptive and non-preemptive variants. In the non-preemptive version, once a process starts executing, it continues until it completes or voluntarily releases the CPU. This leads to a longer overall waiting time for longer processes, known as the “starvation” problem.

In contrast, the preemptive version of SJN allows the currently executing process to be preempted when a shorter job arrives. In this case, the shorter job gets priority, leading to reduced waiting times and improved fairness. However, preemptive SJN scheduling introduces additional overhead due to frequent context switches.

The SJN algorithm is particularly effective for minimizing the average waiting time if the burst times are known accurately. However, in practical scenarios, burst times are often estimated based on historical data or assumptions, which can introduce errors and impact the efficiency of scheduling decisions.

Overall, SJN scheduling aims to optimize system performance by prioritizing the execution of shorter jobs. While it can achieve lower average waiting times, it heavily relies on accurate burst time estimation. The trade-off between estimation accuracy and scheduling performance needs to be carefully considered when implementing SJN in operating systems.

Round Robin (RR) Scheduling

Round Robin (RR) Scheduling is a widely used CPU scheduling algorithm that provides fairness and responsiveness in multitasking operating systems. In RR scheduling, each process is assigned a fixed time slice or quantum of CPU time. The CPU is allocated to the first process in the ready queue and executes for the specified time quantum before being preempted and moved to the back of the queue, allowing the next process in line to execute.

The main idea behind RR scheduling is to provide equal opportunity for each process to execute, ensuring fairness in resource allocation. This fair sharing of the CPU helps prevent a single long-running process from monopolizing the CPU and creating a poor user experience.

One advantage of the RR algorithm is its simplicity and ease of implementation. It ensures that all processes get a chance to execute within a reasonable timeframe, regardless of their execution time requirements.

RR scheduling also provides a degree of responsiveness, especially for interactive tasks. Since each process gets a fixed time quantum, shorter tasks can complete quickly and provide a more interactive experience to users.

However, one drawback of RR scheduling is the potential for high context switch overhead. Context switch occurs when the CPU switches from executing one process to another. With smaller time quantum values, the frequency of context switches increases. This can lead to additional overhead and affect the overall system performance.

Additionally, the choice of time quantum plays a crucial role in the performance of RR scheduling. A short time quantum allows for more frequent process switching, reducing potential delays but increasing the context switch overhead. On the other hand, a long time quantum may lead to longer waiting times for other processes, causing reduced responsiveness.

Overall, Round Robin scheduling is a popular CPU scheduling algorithm due to its fairness and responsiveness. By providing equal opportunity to all processes and preventing a single process from dominating the CPU, RR scheduling ensures a balanced sharing of resources. However, the choice of the time quantum and management of context switch overhead are important considerations in implementing RR scheduling effectively in operating systems.

Priority Scheduling

Priority Scheduling is a widely used CPU scheduling algorithm in operating systems that assigns a priority value to each process. The CPU is allocated to the process with the highest priority at any given time. Priority can be defined based on various factors, such as process importance, user-defined priorities, or process characteristics.

In priority scheduling, each process is associated with a priority value, and the CPU is allocated to the process with the highest priority. If multiple processes have the same priority, they are usually scheduled in a first-come, first-served manner (FCFS) within that priority level.

Priority scheduling allows for fine-grained control over resource allocation, as processes can be assigned different priorities based on their importance or time-criticality. For example, critical system tasks or real-time applications can be assigned high priorities to ensure their timely execution.

There are two common variations of priority scheduling: static priority scheduling and dynamic priority scheduling.

In static priority scheduling, the priority of a process remains constant throughout its life cycle. The priorities are usually assigned based on process characteristics or predetermined rules. This approach provides a stable and predictable scheduling pattern but may not adapt well to changing system conditions.

In dynamic priority scheduling, the priority of a process can change during runtime based on factors such as process behavior, resource usage, or time spent in the system. This adaptive approach helps in managing resource allocation based on changing demands and can enhance system performance and responsiveness.

Priority scheduling, however, presents challenges in preventing starvation, where lower priority processes may not get sufficient CPU time. To address this, some implementations of priority scheduling include aging mechanisms, where the priority of waiting processes gradually increases over time, ensuring a fair distribution of CPU time and preventing starvation.

While priority scheduling provides flexibility and enables the execution of critical processes efficiently, it can also lead to situations where higher priority processes dominate the CPU at the expense of lower priority processes. It is crucial to strike a balance between fairness and responsiveness when setting priorities.

Overall, priority scheduling is a versatile CPU scheduling algorithm that allows for resource allocation based on process importance. By assigning priorities, the algorithm enables the execution of critical tasks and improves system responsiveness. However, careful management of priorities and dealing with potential starvation scenarios are important considerations when implementing priority scheduling in operating systems.

Multilevel Queue Scheduling

Multilevel Queue Scheduling is a CPU scheduling algorithm that divides processes into multiple queues, each with its own priority level or characteristics. Each queue can have its own scheduling algorithm, allowing different classes of processes to be managed separately based on their priority, type, or other properties.

The primary goal of multilevel queue scheduling is to provide differentiated treatment to different categories of processes based on their importance or requirements. By dividing processes into separate queues, system administrators can assign different priorities to different classes of processes, ensuring that critical processes or time-sensitive tasks receive preferential treatment.

The multilevel queue scheduling algorithm allows for efficient handling of different types of processes, such as interactive tasks, batch jobs, or real-time applications, each with their own scheduling requirements. For example, interactive tasks that require quick response times can be assigned to a high-priority queue, while background processes or low-priority tasks can be placed in a separate queue with lower priority.

Each queue can be scheduled using a different scheduling algorithm based on the requirements of the processes in that particular queue. Common scheduling algorithms used within each queue include First-Come, First-Served (FCFS), Round Robin (RR), or Priority Scheduling.

Processes are typically assigned to queues based on predetermined criteria or during the process creation phase. Some systems allow for dynamic movement of processes between queues based on their behavior, priorities, or resource usage during runtime.

One key consideration when implementing multilevel queue scheduling is the order in which the different queues are serviced. This order is usually based on the priority of the queues themselves. For example, high-priority queues may be scheduled before lower-priority queues.

In some multilevel queue scheduling implementations, there may be additional restrictions on the movement of processes between queues. For example, once a process is assigned to a particular queue, it may not be able to move to a higher-priority queue. These restrictions help maintain fairness and prevent lower-priority processes from being starved by higher-priority processes.

In summary, multilevel queue scheduling is a versatile CPU scheduling algorithm that allows for differentiated treatment of processes based on their characteristics and priority. By dividing processes into separate queues and applying different scheduling algorithms, system performance and fairness can be improved. However, careful consideration of queue assignment, scheduling order, and potential restrictions on process movement is necessary to achieve optimal results.

CPU scheduling algorithms are fundamental in managing the allocation of CPU resources in operating systems. They play a crucial role in optimizing system performance, improving responsiveness, and ensuring fair resource allocation among processes. Throughout this article, we have explored various CPU scheduling algorithms and their characteristics, including First-Come, First-Served (FCFS) Scheduling, Shortest Job Next (SJN) Scheduling, Round Robin (RR) Scheduling, Priority Scheduling, and Multilevel Queue Scheduling.

Each CPU scheduling algorithm has its advantages and trade-offs. FCFS provides fairness but may suffer from poor responsiveness. SJN aims to minimize waiting time but relies heavily on accurate burst time estimation. RR provides fairness and responsiveness but may incur high context switch overhead. Priority scheduling allows for fine-grained control over resource allocation but requires careful management to prevent starvation. Multilevel queue scheduling enables differentiated treatment based on priority levels or characteristics of processes.

Choosing the appropriate CPU scheduling algorithm depends on various factors such as system workload, task characteristics, and performance goals. Understanding the strengths and limitations of each algorithm is crucial for designing efficient scheduling strategies in operating systems.

CPU scheduling algorithms continue to evolve as operating systems become more complex and diverse. Innovative approaches and hybrid models are being developed to address specific requirements and optimize system performance. As technology advances, new challenges related to multi-core processors, real-time systems, and dynamic workloads arise, necessitating further advancements in CPU scheduling techniques.

In conclusion, CPU scheduling is a critical aspect of operating systems that directly impacts system performance, responsiveness, and resource utilization. By implementing effective CPU scheduling algorithms, system administrators and developers can optimize the usage of CPU resources, enhance user experience, and achieve efficient multitasking in modern computer systems.

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A Novel Task-to-Processor Assignment Approach for Optimal Multiprocessor Real-Time Scheduling

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Task Scheduling And Processor Assignment

Defining and Managing Financial Projects

Burden Schedule Task Assignment Options

A burden schedule is specified at the project type, project, or task level. When you change the burden schedule, you can propagate the change from the project to all existing tasks. You can change the burden schedule assignment on the project and tasks independently.

Assign the burden schedule to new tasks only

Assign the burden schedule to all tasks

Assign the burden schedule to tasks with previously assigned schedule

Assign Burden Schedule to New Tasks Only

The changed burden schedule is applied to new top tasks only. New subtasks always inherit the burden schedule assignment from the parent task. The burden schedule change doesn't affect the existing tasks or tasks with an existing override.

Assign Burden Schedule to All Tasks

The changed burden schedule is applied to all existing tasks. Existing burden schedule overrides don't change.

Assign Burden Schedule to Tasks with Previously Assigned Schedule

The changed burden schedule applies only to existing tasks that have the same burden schedule that was assigned to the project before this change. The changed burden schedule isn't applied on tasks with an existing burden schedule override.

Purdue men's basketball will take on these teams at home and on the road in Big Ten play

The Purdue men's basketball team learned more about its 2024-25 season after the Big Ten announced conference opponents Wednesday.

Purdue will play Indiana, Michigan and Rutgers in a home-and-away series.

For single-play home games, the Boilermakers will face off with Maryland, Nebraska, Northwestern, Ohio State, UCLA, USC and Wisconsin.

Illinois, Iowa, Michigan State, Minnesota, Oregon, Penn State and Washington make up the single-play road game schedule for Purdue.

Dates, times and television assignments will come at a later date, according to a release.

This coming season will mark the first time UCLA, USC, Oregon and Washington will be in the Big Ten. Over the previous six years with a 20-game schedule, the Boilermakers are 84-35, which is the league's best record during that time.

Last season, Purdue went 34-5 and won the Big Ten regular-season championship for the second straight year and reached the National Championship game.

Recruiting: What Matt Painter said about the 6 players in Purdue's top-10 ranked 2024 recruiting class

The Boilermakers carry an 11-game home winning streak against conference opponents into the 2024-25 season. UCLA will visit Mackey Arena for the first time since 2000. The Bruins and Boilermakers played in Mackey Arena's first game on Dec. 6, 1967 (UCLA, along with Lew Alcindor, Purdue and Rick Mount, 73-71).

The trip to Washington for Purdue will mark the first time the Boilermakers have traveled to Seattle since 1967. Purdue last played in Oregon in 1988.

Is there a NASCAR race today? The NASCAR TV schedule for Kansas this weekend

Yet another NASCAR tripleheader is on the docket this weekend with Kansas Motor Speedway a 1½-mile tri-oval next up on the season schedule.

Last week, Denny Hamlin padded his career and season stats, scoring his 54th victory and third of the year by holding off Kyle Larson over the closing laps at Dover Motor Speedway . Hamlin is also the defending winner of this weekend's Cup Series event, the AdventHealth 400 and Toyota has claimed the last four victories at Kansas, five of the last six and seven of the last nine.

A pair of races are set for Saturday with the ARCA Menards Series Tide 150 (2 p.m.) and the Craftsman Truck Series Heart of America 200 (8 p.m.) both scheduled for green flags. The Xfinity Series will take the week off.

Here is a full NASCAR schedule of events at Kansas this weekend along with television broadcast assignments:

NASCAR BETTING: NASCAR odds, picks, predictions and DFS lineup advice for Kansas. Who's the best bet?

NASCAR TV schedule this weekend

Friday, May 3

10 a.m.: ARCA Menards Series open practice (No TV)

Saturday, May 4

10:25 a.m.: ARCA Menards Series practice (No TV)
11:10: ARCA Menards Series qualifying (No TV)
12:05 p.m.: Craftsman Truck Series practice (FS1)
12:35: Craftsman Truck Series qualifying (FS1)
2: ARCA Menards Series, Tide 150 (FS1)
5:05: Cup Series practice (FS1)
5:50: Cup Series qualifying (FS1)
8: Craftsman Truck Series, Heart of America 200 (FS1)

Sunday, May 5

3 p.m.: Cup Series, AdventHealth 400 (FS1)

White Sox Designate Former Dodgers Outfielder for Assignment

Pillar is searching for a new club...

Author: Jason Fray

In this story:

Baseball can be a cruel game.

Kevin Pillar knows this all too well. The journeyman outfielder has suited up for eight different ballclubs throughout his career (Toronto Blue Jays, San Francisco Giants, Los Angeles Dodgers, Atlanta Braves, Chicago White Sox, New York Mets, Colorado Rockies and Boston Red Sox).

Unfortunately, he'll be looking for club No. 9.

Kevin Pillar designated for assignment so White Sox can make room for Tommy Pham, sources tell @TheAthletic . — Ken Rosenthal (@Ken_Rosenthal) April 26, 2024

Ken Rosenthal reported that Pillar will be designated for assignment by the Chicago White Sox. The plan for the Southsiders is to sign fellow veteran OF Tommy Pham.

Pillar spoke about his status with Foul Territory Show :

Pillar elaborated further:

Pillar was hitting .160 with one home run and four RBIs for the White Sox over the course of 17 games and 32 at-bats.

He previously played for the Dodgers in 2022 -- making appearances in only four games before fracturing his shoulder in a game versus the Pirates.

Pillar, 35, grew up in the Los Angeles suburb of West Hills.

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10.2: Multiprocessor Scheduling
Scheduling is not as straight forward as it was with the single processor, the algorithms are more complex due to the nature of multiprocessors being present. There are several different concepts that have been studied and implemented for multiprocessor thread scheduling and processor assignment. A few of these concepts are discussed below ...
PDF Systems: Multiprocessor, Multicore and Real-Time Scheduling
Dedicated Processor Assignment Dynamic Scheduling Four approaches for multiprocessor thread scheduling and processor assignment are: a set of related threads scheduled to run on a set of processors at the same time, on a one-to-one basis processes are not assigned to a particular processor provides implicit scheduling defined by the assignment of
PDF COS 318: Operating Systems CPU Scheduling
Scheduling Criteria. u Assumptions. l One process per user and one thread per process. l Processes are independent. u Goals for batch and interactive systems. l Provide fairness. l Everyone makes some progress; no one starves. l Maximize CPU utilization • Not including idle process. l Maximize throughput • Operations/second (min overhead ...
PDF CPU Scheduling
4. ready to execute (in ready state), and allocates the CPU to one of them (puts in running state). CPU scheduling can be non-preemptive or pre-emptive. Non-preemptive scheduling decisions may take place when a process changes state: switches from running to waiting state. switches from running to ready state. switches from waiting to ready.
PDF Multiprocessor and Real-Time Scheduling
Dynamic Scheduling Number of threads in a process changes dynamically (by the application) Operating system adjusts the processor load using some of these strategies: assign idle processors to new threads new arrivals may be assigned to a processor by taking away a processor from some other application that uses > 1 processor
PDF CSC 553 Operating Systems
scheduling and processor assignment are: - Load Sharing - processes are not assigned to a particular processor - Gang Scheduling - a set of related threads scheduled to run on a set of processors at the same time, on a one-to-one basis - Dedicated Processor Assignment - provides implicit scheduling defined by the assignment of threads to ...
PDF Chapter 10 Multiprocessor scheduling
As an extreme form of the gang scheduling, the dedicated processor assignment strategy is the opposite of the load sharing one. Each program is allocated a number of processors equal to that of the threads in the program, for the duration of the program execution. With dynamic scheduling, the number of threads in a process can be altered during ...
PDF Chapter 10 Multiprocessor scheduling
With gang scheduling, a set of related threads is scheduled to run on a set of processors at the same time, on a one-to-one basis. Then, syn-chronization blocking may be reduced, hence, less process switching and less overhead. As an extreme form of the gang scheduling, the dedicated processor assignment strategy is the opposite of the load ...
PDF COS 318: Operating Systems CPU Scheduling
lProcess/thread to processor assignment uGang scheduling (co-scheduling) lThreads of the same process will run together lProcesses of the same application run together uDedicated processor assignment lThreads will be running on specific processors to completion lOn a multiprocessor it is called affinity (or CPU pinning)
14.2: Scheduling Algorithms
In packet-switched computer networks and other statistical multiplexing, the notion of a scheduling algorithm is used as an alternative to first-come first-served queuing of data packets. The simplest best-effort scheduling algorithms are round-robin, fair queuing (a max-min fair scheduling algorithm), proportionally fair scheduling and maximum ...
CS401: Operating Systems, Topic: Unit 4: CPU Scheduling
Unit 4: CPU Scheduling. Central Process Unit (CPU) scheduling deals with having more processes/threads than processors to handles those tasks, meaning how the CPU determines which jobs it is going to handle in what order. A good understanding of how a CPU scheduling algorithm works is essential to understanding how an Operating System works; a ...
Multiprocessor and Distributed Real-Time Scheduling
The first step in the partitioning scheduling is the assignment of tasks to processors of the multiprocessor system. This step is followed by implementing a uniprocessor scheduling algorithm in each processor. ... Oh DI, Baker TP (1998) Utilization bounds for N-processor rate monotone scheduling with static processor assignment. Real Time Syst ...
Multiple-Processor Scheduling in Operating System
Approaches to Multiple-Processor Scheduling -. One approach is when all the scheduling decisions and I/O processing are handled by a single processor which is called the Master Server and the other processors executes only the user code. This is simple and reduces the need of data sharing. This entire scenario is called Asymmetric ...
CPU Scheduling Algorithms in Operating Systems
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 ...
CPU Scheduling in Operating Systems
Scheduling of processes/work is done to finish the work on time. CPU Scheduling is a process that allows one process to use the CPU while another process is delayed (in standby) due to unavailability of any resources such as I / O etc, thus making full use of the CPU. The purpose of CPU Scheduling is to make the system more efficient, faster, and fairer.
What Is CPU Scheduling
CPU scheduling is a fundamental component of modern operating systems and is the mechanism that determines which tasks or processes get access to the CPU's processing time. In simpler terms, it is the algorithmic strategy used to schedule the execution of processes in a computer system. ... However, careful consideration of queue assignment ...
A Novel Task-to-Processor Assignment Approach for Optimal
Recent decades have recognized the popularization of multiprocessor architectures in real-time embedded systems. Real-time task scheduling in such systems has become a challenging problem as a result. In this paper, we are presenting an optimal scheduling algorithm, which can successfully schedule any task sets with no deadline miss if the total utilization of tasks does not exceed the ...
PDF Operating Systems 2014 Assignment 2: Process Scheduling
1 Introduction. Process scheduling is an important part of the operating system and has influence on the achieved CPU utilization, system throughput, waiting time and response time. Especially for real-time and modern interactive systems (such as smart phones), the scheduler must be tuned to perfection. The task of the scheduler is to decide ...
Scheduling DAGs When Processor Assignments Are Specified
Scheduling DAGs When Processor Assignments Are Specified. Pages 111-116. Previous Chapter Next Chapter. ABSTRACT. The problem of scheduling a workload represented as a directed acyclic graph (DAG) upon a dedicated multiprocessor platform is considered, in which each individual vertex of the DAG is assigned to a specific processor and the ...
Task Scheduling And Processor Assignment
Task Scheduling And Processor Assignment An important factor in the performance of a parallel system, is how the computational load is mapped onto the compute nodes in the system. Ideally, to achieve maximum parallelism, the load must be evenly distributed across the compute nodes.
Burden Schedule Task Assignment Options
When you change the burden schedule, you can propagate the change from the project to all existing tasks. You can change the burden schedule assignment on the project and tasks independently. Assign the burden schedule to new tasks only. Assign the burden schedule to all tasks. Assign the burden schedule to tasks with previously assigned schedule.
Spring 2024 Camden Exam Packet
Final Exam schedule/information/room assignments. At the top of this page is a link to Detailed Exam Information, which contains all exam information, including - whether the exam will be in-class or web-administered, whether it will be open or closed book, which classroom an exam will be administered in, and so forth.
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Illinois, Iowa, Michigan State, Minnesota, Oregon, Penn State and Washington make up the single-play road game schedule for Purdue. Dates, times and television assignments will come at a later ...
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PDF CPU scheduling basics COS 318: Operating Systems CPU Scheduling
CPU Scheduler. Selects from among the processes/threads that are ready to execute (in ready state), and allocates the CPU to one of them (puts in running state). CPU scheduling can be non-preemptive or pre-emptive. Non-preemptive scheduling decisions may take place when a process changes state: switches from running to waiting state.
NASCAR schedule today: Cup, Truck and ARCA events on TV this weekend
Here is a full NASCAR schedule of events at Kansas this weekend along with television broadcast assignments: ... NASCAR TV schedule this weekend. Friday, May 3. 10 a.m.: ARCA Menards Series open ...
White Sox Designate Former Dodgers Outfielder for Assignment
Unfortunately, he'll be looking for club No. 9. Kevin Pillar designated for assignment so White Sox can make room for Tommy Pham, sources tell @TheAthletic. Ken Rosenthal reported that Pillar will ...

14.2: Scheduling Algorithms

Scheduling Algorithms

First come, first served

Shortest remaining time first

Fixed priority pre-emptive scheduling

Round-robin scheduling

Multilevel queue scheduling

Work-conserving schedulers

Choosing a scheduling algorithm

CS401: Operating Systems

4.1: Scheduling General Objective

4.4: Algorithms

Unit 4 Exercises and Assessment

Multiprocessor and Distributed Real-Time Scheduling

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CPU Scheduling Algorithms in Operating Systems

What is CPU Scheduling?

Types of CPU Scheduling

Preemptive Scheduling

Non-Preemptive Scheduling

When scheduling is Preemptive or Non-Preemptive?

Important CPU scheduling Terminologies

CPU Scheduling Criteria

Interval Timer

What is Dispatcher?

Types of CPU scheduling Algorithm

Shortest Remaining Time

First Come First Serve

Characteristics of FCFS method

Characteristics of SRT scheduling method

Priority Based Scheduling

Round-Robin Scheduling

Characteristics of Round-Robin Scheduling

Shortest Job First

Characteristics of SJF Scheduling

Multiple-Level Queues Scheduling

Characteristic of Multiple-Level Queues Scheduling

The Purpose of a Scheduling algorithm

Structure of Operating System

Types of OS

Process Management

CPU Scheduling in OS

Threads in OS

Process Synchronization

Critical Section Problem Solution

Deadlocks & Deadlock Handling Methods

Memory Management

Page Replacement Algorithms

OS Interview Questions

OS Quiz and GATE PYQ's

What is Process Scheduling?

What is a process?

How is process memory used for efficient operation?

What is the need for CPU scheduling algorithm?

Objectives of Process Scheduling Algorithm:

What are the different terminologies to take care of in any CPU Scheduling algorithm?

Things to take care while designing a CPU Scheduling algorithm?

What are the different types of CPU Scheduling Algorithms?

1. First Come First Serve:

2. Shortest Job First(SJF):

3. Longest Job First(LJF):

4. Priority Scheduling:

5. Round robin:

6. Shortest Remaining Time First:

7. Longest Remaining Time First:

8. Highest Response Ratio Next:

9. Multiple Queue Scheduling:

10. Multilevel Feedback Queue Scheduling: :

Comparison between various CPU Scheduling algorithms

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