{"id":11001,"date":"2025-11-09T03:32:42","date_gmt":"2025-11-09T03:32:42","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=11001"},"modified":"2025-11-09T03:32:42","modified_gmt":"2025-11-09T03:32:42","slug":"understanding-cpu-scheduling-algorithms-from-fcfs-to-round-robin","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/understanding-cpu-scheduling-algorithms-from-fcfs-to-round-robin\/","title":{"rendered":"Understanding CPU Scheduling Algorithms: From FCFS to Round Robin"},"content":{"rendered":"<h1>Understanding CPU Scheduling Algorithms: From FCFS to Round Robin<\/h1>\n<p>The heart of a computing system lies in its CPU, which executes tasks assigned by operating systems. Efficiently allocating CPU time to various processes is critical for performance, and this is where CPU scheduling algorithms come into play. This article will explore several fundamental CPU scheduling algorithms, including First-Come, First-Served (FCFS), Shortest Job Next (SJN), Priority Scheduling, and Round Robin (RR), detailing their characteristics, advantages, and disadvantages.<\/p>\n<h2>1. What is CPU Scheduling?<\/h2>\n<p>CPU scheduling is a process that allows an operating system to determine which process should be allocated CPU time at any given moment. This can involve a mix of different algorithms, each with its own approach to optimizing CPU efficiency and ensuring fair resource allocation among processes.<\/p>\n<h2>2. First-Come, First-Served (FCFS)<\/h2>\n<p>First-Come, First-Served (FCFS) is one of the simplest and most straightforward CPU scheduling algorithms. In this approach, the process that arrives first in the ready queue is the first to be executed. It&#8217;s akin to waiting in line at a bakery; the first customer gets served first.<\/p>\n<h3>Characteristics of FCFS:<\/h3>\n<ul>\n<li><strong>Non-preemptive:<\/strong> Once a process is allocated the CPU, it cannot be interrupted until it completes.<\/li>\n<li><strong>Easy to implement:<\/strong> Requires minimal overhead and is simple to program.<\/li>\n<li><strong>Time Complexity:<\/strong> O(n) for n processes.<\/li>\n<\/ul>\n<h3>Advantages of FCFS:<\/h3>\n<ul>\n<li>Simple to understand and implement.<\/li>\n<li>No starvation: Every process gets executed eventually.<\/li>\n<\/ul>\n<h3>Disadvantages of FCFS:<\/h3>\n<ul>\n<li>Long waiting times can occur, especially if a long process arrives before shorter ones.<\/li>\n<li>Does not utilize CPU efficiently (convoy effect), impacting overall system performance.<\/li>\n<\/ul>\n<h3>Example of FCFS:<\/h3>\n<p>Consider three processes with their arrival and burst times:<\/p>\n<ul>\n<li>Process 1: Arrival Time = 0, Burst Time = 4<\/li>\n<li>Process 2: Arrival Time = 1, Burst Time = 3<\/li>\n<li>Process 3: Arrival Time = 2, Burst Time = 1<\/li>\n<\/ul>\n<p>The scheduling order will be:<\/p>\n<pre>\nProcess 1 (0-4) \u2794 Process 2 (4-7) \u2794 Process 3 (7-8)\n<\/pre>\n<p>Average waiting time = (0 + 3 + 5) \/ 3 = 2.67 ms<\/p>\n<h2>3. Shortest Job Next (SJN)<\/h2>\n<p>Shortest Job Next (SJN), also known as Shortest Job First (SJF), schedules processes based on the length of their burst times. The process with the shortest burst time is selected next, leading to reduced waiting time overall.<\/p>\n<h3>Characteristics of SJN:<\/h3>\n<ul>\n<li><strong>Non-preemptive:<\/strong> Once a process starts executing, it cannot be preempted.<\/li>\n<li><strong>Dynamic:<\/strong> The scheduler needs to know the burst times of jobs.<\/li>\n<\/ul>\n<h3>Advantages of SJN:<\/h3>\n<ul>\n<li>Minimizes average waiting time.<\/li>\n<li>Provides optimal scheduling for a given set of processes.<\/li>\n<\/ul>\n<h3>Disadvantages of SJN:<\/h3>\n<ul>\n<li>Involves complicated calculations as it requires knowledge of future process times.<\/li>\n<li>Can lead to starvation for longer processes.<\/li>\n<\/ul>\n<h3>Example of SJN:<\/h3>\n<p>Given processes:<\/p>\n<ul>\n<li>Process 1: Arrival Time = 0, Burst Time = 8<\/li>\n<li>Process 2: Arrival Time = 1, Burst Time = 4<\/li>\n<li>Process 3: Arrival Time = 2, Burst Time = 9<\/li>\n<\/ul>\n<p>The scheduling order will be:<\/p>\n<pre>\nProcess 2 (1-5) \u2794 Process 1 (5-13) \u2794 Process 3 (13-22)\n<\/pre>\n<p>Average waiting time = (4 + 5 + 11) \/ 3 = 6.67 ms<\/p>\n<h2>4. Priority Scheduling<\/h2>\n<p>Priority Scheduling allocates CPU time to processes based on priority, where each process is assigned a precedence level. The process with the highest priority is executed first. There are two types: preemptive and non-preemptive.<\/p>\n<h3>Characteristics of Priority Scheduling:<\/h3>\n<ul>\n<li><strong>Both:<\/strong> Can be implemented in preemptive or non-preemptive styles.<\/li>\n<li><strong>Dynamic:<\/strong> Priority levels can change based on criteria.<\/li>\n<\/ul>\n<h3>Advantages of Priority Scheduling:<\/h3>\n<ul>\n<li>Flexible allocation based on important tasks.<\/li>\n<li>Useful in real-time systems.<\/li>\n<\/ul>\n<h3>Disadvantages of Priority Scheduling:<\/h3>\n<ul>\n<li>Can lead to starvation for low-priority processes.<\/li>\n<li>Determining priority levels can be complex.<\/li>\n<\/ul>\n<h3>Example of Priority Scheduling:<\/h3>\n<p>Consider processes:<\/p>\n<ul>\n<li>Process 1: Arrival Time = 0, Burst Time = 5, Priority = 2<\/li>\n<li>Process 2: Arrival Time = 1, Burst Time = 3, Priority = 1<\/li>\n<li>Process 3: Arrival Time = 2, Burst Time = 1, Priority = 3<\/li>\n<\/ul>\n<p>The scheduling order will be:<\/p>\n<pre>\nProcess 2 (1-4) \u2794 Process 1 (4-9) \u2794 Process 3 (9-10)\n<\/pre>\n<p>Average waiting time = (0 + 3 + 7) \/ 3 = 3.33 ms<\/p>\n<h2>5. Round Robin (RR)<\/h2>\n<p>Round Robin (RR) is one of the most widely used CPU scheduling algorithms in time-sharing systems. Each process is allowed a small unit of time (called a time quantum) to execute, after which it is preempted and the CPU is allocated to the next process in the queue.<\/p>\n<h3>Characteristics of Round Robin:<\/h3>\n<ul>\n<li><strong>Preemptive:<\/strong> Processes are interrupted and rescheduled based on a time quantum.<\/li>\n<li><strong>Fairness:<\/strong> Allows each process to receive a fair share of CPU time.<\/li>\n<\/ul>\n<h3>Advantages of Round Robin:<\/h3>\n<ul>\n<li>Good response time for user-interactive systems.<\/li>\n<li>Prevents starvation of processes.<\/li>\n<\/ul>\n<h3>Disadvantages of Round Robin:<\/h3>\n<ul>\n<li>Performance can degrade with larger time quanta, leading to higher waiting times.<\/li>\n<li>Overhead increases with context switching when time quantum is too small.<\/li>\n<\/ul>\n<h3>Example of Round Robin:<\/h3>\n<p>Given processes:<\/p>\n<ul>\n<li>Process 1: Arrival Time = 0, Burst Time = 8<\/li>\n<li>Process 2: Arrival Time = 1, Burst Time = 4<\/li>\n<li>Process 3: Arrival Time = 2, Burst Time = 9<\/li>\n<\/ul>\n<p>Assuming a time quantum of 3 time units, the order of execution will be:<\/p>\n<pre>\nProcess 1 (0-3) \u2794 Process 2 (3-6) \u2794 Process 1 (6-8) \u2794 Process 3 (8-11) \u2794 Process 2 (11-12) \u2794 Process 1 (12-14) \u2794 Process 3 (14-22)\n<\/pre>\n<p>Average waiting time = (14 + 6 + 12) \/ 3 = 10.67 ms<\/p>\n<h2>Conclusion<\/h2>\n<p>Understanding CPU scheduling algorithms is fundamental for developers who want to enhance the performance of operating systems and applications. By choosing the right scheduling algorithm based on the process requirements and overall system objectives, developers can vastly improve system efficiency and user experience.<\/p>\n<p>Each method has its strengths and weaknesses; hence the choice of an algorithm depends on the specific use case and priorities, such as minimizing waiting time, improving responsiveness, or ensuring fairness among processes. By leveraging these scheduling strategies, developers can optimize the CPU&#8217;s capabilities to better serve their applications and users.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Understanding CPU Scheduling Algorithms: From FCFS to Round Robin The heart of a computing system lies in its CPU, which executes tasks assigned by operating systems. Efficiently allocating CPU time to various processes is critical for performance, and this is where CPU scheduling algorithms come into play. This article will explore several fundamental CPU scheduling<\/p>\n","protected":false},"author":145,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[1143,249],"tags":[1177,1249,1165,1174,1176],"class_list":{"0":"post-11001","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-cpu-scheduling","7":"category-operating-systems","8":"tag-cpu","9":"tag-operating-systems","10":"tag-process","11":"tag-scheduler","12":"tag-scheduling"},"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/11001","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/users\/145"}],"replies":[{"embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/comments?post=11001"}],"version-history":[{"count":1,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/11001\/revisions"}],"predecessor-version":[{"id":11002,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/11001\/revisions\/11002"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=11001"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=11001"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=11001"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}