Job Scheduling Algorithms

Job scheduling algorithms are critical to optimizing the execution of tasks on a CPU in computer science. These algorithms determine the order in which jobs are executed to improve efficiency, reduce waiting times, and enhance overall system performance. But even more important than the algorithms you choose for your workflow is the tools you equip yourself with to enable success.

  • What are the 4 major job scheduling algorithms? 
  • The Best Job Scheduling Algorithm for Each Operating System
    • Mac
    • Windows
    • Linux
  • Managing Job Scheduling Algorithms with Redwood
  • Job Scheduling Algorithms FAQs

What are the 4 major job scheduling algorithms?

The four major job scheduling algorithms are First Come First Served (FCFS), Shortest Job First (SJF), Round Robin scheduling (RR), and priority scheduling. Each algorithm has its own characteristics and is suitable for specific scenarios.

First Come First Served (FCFS) is a job scheduling algorithm that executes jobs based on their arrival time to the system. This algorithm is an example of non-preemptive scheduling and implementation is based on FIFO. First Come First Served is popular because of its simplicity, but it has downsides like longer average waiting times for short jobs that can cause scheduling problems.

Shortest Job First (SJF) prioritizes jobs that can be processed in the shortest amount of time. SJF can help achieve faster turnaround times, especially for short jobs, but this method requires accurate estimates for job runtime and processing time for optimal performance.

In Round Robin scheduling, each job has a defined time slice, and the sequence of jobs cycles through in a round robin manner. RR scheduling can be helpful for managing large job quantities but can have lower throughput. Round robin scheduling is preemptive and uses context switching for saving preemption process states.

The priority scheduling algorithm works by assigning jobs a priority, such as high priority, lower priority, or highest priority. Jobs are then executed in a priority queue. If jobs have the same priority, they’re executed on a first come first serve basis. Priority scheduling is non-preemptive and works well for batch process scheduling

First Come First Served (FCFS) vs Shortest Job First (SJB) 

When comparing the most widely-used two job scheduling algorithms: First Come First Served (FCFS) and Shortest Job First (SJF), there are a number of elements to consider, including advantages, and limitations. 

FCFS and SJF are both non-preemptive algorithms, but they differ in prioritizing job execution. First Come First Served schedules new jobs by arrival time, while Shortest Job First prioritizes jobs with the shortest execution time. 

Advantages of FCFS include simplicity and fair job execution, while SJB excels in efficient allocation and execution of short jobs. Because short jobs receive priority with SJB, they experience improved throughput and reduced average waiting times. 

Limitations of the First Come First Served job scheduling algorithm present in longer average waiting times and poor turnaround time. If a long job arrives first in a ready state, all subsequent jobs have to wait for its completion. This can cause significant turnaround times for jobs that otherwise could have been executed quickly in the shortest remaining time.

Conversely, a limitation associated with Shortest Job First is the possibility of starvation. If shorter jobs are continuously entering the system, longer jobs may end up sitting there until it’s too late. Optimal performance of the Shortest Job First algorithm is dependent on accurate job duration estimation, which aren’t always available or require internal resources to obtain. This can cause a strain on the computer science team if they have to spend time creating data structures for the algorithm to run properly. 

The Best Job Scheduling Algorithms for Each Operating System 

Deciding which job scheduling algorithm to use depends on a variety of factors, including job requirements, programming language like JavaScript or HTML, system architecture, development resources, and more. Some algorithms work better with certain technologies and operating systems. 

Best Job Scheduling Algorithm for Mac Operating System

One of the commonly employed scheduling algorithms on Mac operating systems is the Multilevel Feedback Queue (MLFQ) algorithm. MLFQ is often used to optimize the execution time of tasks on the CPU. The Multilevel Feedback Queue algorithm is a dynamic priority scheduling algorithm that categorizes jobs into multiple priority queues. The MLFQ algorithm allows for jobs to move between queues based on runtime, processing time, and priority.

In a Mac operating system, the MLFQ algorithm can handle a mix of interactive tasks, background tasks, and real-time tasks effectively by assigning appropriate priorities. This algorithm ensures that interactive tasks, which require fast response times, are given higher priority, while background tasks and less time-sensitive jobs are assigned lower priorities. This approach helps maintain system responsiveness, prompt completion time for tasks, and fast response time for new jobs.

Best Job Scheduling Algorithm for Windows Operating System

In the Windows operating system, the default job scheduling algorithm used is the Multilevel Feedback Queue algorithm. Windows also supports a subset of scheduling algorithms through its Windows Scheduler API, including First Come First Served (FCFS), Shortest Job First (SJF), Round Robin, and priority scheduling. 

The Windows Scheduler API allows developers and system administrators to customize job scheduling algorithms based on specific job requirements.

Best Job Scheduling Algorithm for Linux Operating System

Choosing the best job scheduling algorithm for Linux depends on the specific Linux distribution, kernel version, and requirements of the system. A widely-used and highly regarded scheduling algorithm for Linux is the Completely Fair Scheduler (CFS).

The Completely Fair Scheduler is a process scheduler that fairly distributes CPU time among active processes. It utilizes a red-black tree data structure to maintain a queue of runnable processes, allowing for programmatic scheduling decisions. The CFS algorithm considers factors like process priority, virtual runtime, and load balancing to ensure fair distribution and optimal resource utilization.

There are other scheduling algorithms and options compatible with Linux. These include the Real-Time Scheduler (SCHED_FIFO), which prioritizes real-time tasks, and the Deadline Scheduler (SCHED_DEADLINE), which guarantees timely execution for tasks with strict deadlines. 

Managing Job Scheduling Algorithms with Redwood 

Redwood is a powerful tool for managing job scheduling algorithms, providing developers the flexibility to customize and optimize their workflows. With Redwood, it’s possible to easily implement various scheduling algorithms based on specific requirements.

One of the advantages of Redwood is support for priority scheduling. This feature is useful in real-time systems where certain jobs require immediate attention or have strict deadlines.

Additionally, Redwood offers multilevel feedback queues, enabling the execution of jobs in multiple priority levels. Teams can easily move jobs between queues based on processing time, priority, and runtime. This feature provides a balance between short and long jobs, preventing starvation and optimizing system performance.

Job Scheduling Algorithms Frequently Asked Questions

What is the difference between a single-processor and a multiprocessor system?

A single-processor system has only one CPU that executes jobs one at a time. In contrast, a multiprocessor system has multiple CPUs, allowing concurrent execution of jobs. Multiprocessor systems can achieve higher throughput and better performance by utilizing parallel processing.

Learn how to easily manage cross platform scheduling and batch jobs with RunMyJobs by Redwood

What are the advantages of an online scheduler?

An online scheduler is a scheduling algorithm that makes decisions dynamically as jobs arrive. Some advantages of online schedulers include adaptability to varying workloads, efficient resource utilization, and the ability to handle real-time tasks effectively. Online schedulers can adjust to changing circumstances and optimize scheduling decisions based on the current state of the real-time system.

See how teams automate job scheduling on any platform using Redwood for workload automation.