Shortest Seek Time First Scheduling

Duration: 12 min

This video lesson is available to enrolled students.

Return to /learn/BPSC/operating-systems/disc-scheduling/fcfs-sstf-scheduling/asset-shortest-seek-time-first-scheduling after enrolling

Inside: a video lesson and guided study material.

Module outline

  1. About the Course: About the Course
  2. Section 1 | Language (Hindi): भाषा, लिपि और वर्ण, वर्तनी, वर्ण-विन्यास, अनुस्वार, अनुनासिक, चंद्रबिंदु और ‘र’ के रूप, शब्द, पद और शब्द-भेद, संज्ञा और सर्वनाम, पद-भेद और रूप-विकार, लिंग, वचन, पुरुष और कारक, काल, वाच्य, वाक्य और वाक्य-भेद, संधि, समास, विलोम शब्द, पर्यायवाची शब्द, सामान्य अशुद्धियाँ, अनेक शब्दों के लिए एक शब्द, शब्द-भंडार, अपठित बोध, Multi-Topic Content (Cross-Concept Material), वर्णमाला
  3. Section 1 | Language (उर्दू भाषा — 22 प्रश्न (निश्चित)): رسم الخط، حروفِ تہجی اور املا / Script, Alphabet and Spelling, اسم، ضمیر اور صفت / Nouns, Pronouns and Adjectives, الفاظ، محاورے اور ضرب الامثال / Vocabulary, Idioms and Proverbs, تفہیمِ عبارت، انشا اور ترجمہ / Comprehension, Writing and Translation, Comprehension
  4. Section 1 | Language (বাংলা / बांग्ला भाषा — 22 प्रश्न (निश्चित)): শব্দার্থ, বাগধারা ও ভাষাশুদ্ধি / Meaning, Idioms and Correct Usage, বাক্য ও বাচ্য / Sentences and Voice, শব্দগঠন ও শব্দভান্ডার / Word Formation and Vocabulary, অনুবাদ, বোধপরীক্ষণ ও রচনা / Translation, Comprehension and Composition
  5. Section 1 | Language (English): Vocabulary, Noun, Subject Verb Agreement (Verb Noun Agreement), Adjectives, Verbs and Tenses, Pronoun, Verb, Adverb, Prepositions and Conjunctions, Parts of Speech, Determiner, Conjunction, Modals, Transformation of Sentences, Direct and Indirect Speech, Sentence Correction (Error Correction), Sentence Completion (Fill in the blanks), Sentence Construction, Sentence Structure and Common Errors, Reading Comprehension, Cloze Test (Cloze Passage), Verbal Analogy (Word Based Analogy), Facts, Inference and Judgements
  6. Section 2 | Primary Maths & Reasoning: अनुपात और समानुपात / Ratio and Proportion, Divisibility Rules, मिश्रण और एलिगेशन / Mixture and Alligation, Permutation and Combination, संख्या पद्धति / Number System, महत्तम समापवर्तक और लघुत्तम समापवर्त्य / HCF and LCM, Sequence and Series (Series), समय, चाल और दूरी / Time, Speed and Distance, श्रेणी / Series, कूट लेखन एवं कूट वाचन / Coding-Decoding, अशाब्दिक तर्क / Non-Verbal Reasoning, प्रतिशत / Percentage, औसत / Average, लाभ, हानि और बट्टा / Profit, Loss and Discount, साधारण और चक्रवृद्धि ब्याज / Simple and Compound Interest, समय और कार्य / Time and Work, घन क्षेत्रमिति / Solid Mensuration, Age Problems, Arithmetic, सरलीकरण, घातांक और करणी / Simplification, Indices and Surds, Algebra, Probability, Trigonometry, Syllogisms, बैठक व्यवस्था एवं पहेलियाँ / Seating Arrangement and Puzzles, रक्त संबंध / Blood Relations, दिशा एवं दूरी / Direction and Distance, Analytical Reasoning (Counting Figures Reasoning), गणितीय संक्रियाएँ एवं लुप्त संख्या / Mathematical Operations and Missing Numbers, कैलेंडर एवं घड़ी / Calendar and Clock, Clock, घन, पासा एवं आकृतियों की गिनती / Cubes, Dice and Figure Counting, वेन आरेख, न्याय वाक्य एवं असमानता / Venn Diagrams, Syllogism and Inequalities, Data Arrangement, Inequality, सादृश्यता एवं वर्गीकरण / Analogy and Classification, क्रम एवं रैंकिंग / Order and Ranking, समुच्चय, संबंध, फलन और विविक्त गणित / Sets, Relations, Functions and Discrete Mathematics, संख्या पद्धति और संख्या सिद्धांत / Number Systems and Number Theory, निर्देशांक ज्यामिति, सदिश और त्रिविमीय ज्यामिति / Coordinate Geometry, Vectors and 3D Geometry, बहुपद, समीकरण और सम्मिश्र संख्याएँ / Polynomials, Equations and Complex Numbers
  7. Section 2 | General Awareness (GK + Current Affairs): Polity, वैज्ञानिक उपलब्धियाँ और अनुसंधान / Scientific Achievements and Research, Biology, राज्य विशेष सामान्य ज्ञान / State-Specific General Knowledge, Chemistry, History, Geography, Indian Economy, Environment, समसामयिकी / Current Affairs, Static General Knowledge
  8. Section 3 | Computer System and Org. (CSO): Basics of COA, Floating Point Rep, Cache Memory Organization, Instr Formats & Modes, Programming the Basic Computer, Input Output Organisation, Register Transfer and Microoperations, Control Unit Design
  9. Section 3 | Database Management System: Basics of DBMS, ER Diagram, Relational Model & Functional Dependencies, Keys & Integrity Constraints, Normalization (1NF - BCNF), Decomposition Properties & 4NF, SQL, File Organization & Indexing, Relational Algebra, Relational Calculus, Transaction Management, Concurrency Control
  10. Section 3 | Operating System: Introduction to OS, Process Management, CPU Scheduling, Threads & Process Creation, Memory Management, Virtual Memory, Disc Scheduling, File Management, Process Synchronization
  11. Section 3 | Computer Networks: Introduction to CN, Data Communication, DLL: Access Control, DLL: Flow Control, DLL: Error Control, Data Link Layer - Ethernet, Net Layer: IPv4 & Proto, Net Layer: IP Addressing, Transport Layer Services, TL: Congestion & UDP, Application Layer, Hardware basics
  12. Section 3 | Data Structures: Introduction to DS, Array, Stack, Queue, Linked List, Tree, Graphs, Hashing, Algorithm Analysis, Time Complexity Analysis, Sorting Algorithms, Greedy Algorithms, Minimum Spanning Trees, Shortest Path Algos
  13. Section 3 | Theory Of Computation & Compiler Design: Introduction to TOC, Deterministic FA (DFA), Non-Deterministic FA, Regular Expressions, Grammar, Regular Language Properties, Moore & Mealy Machines, Pushdown Automata & CFG, Turing Machines, Intro to Compilers, Lexical Analysis, Grammar & CFG, Syntax Analysis: Top-Down, Syntax Analysis: Bottom-Up, Semantic Analysis & SDT, Intermediate Code Gen, Code Optimization
  14. Section 3 | Emerging Trends (AI, Societal Ethics & MS Office): कंप्यूटर के मूल सिद्धांत / Computer Fundamentals, Emerging Tech Trends, Society, Law & Ethics, MS Office Applications, व्यापार, विपणन एवं उपभोक्ता संरक्षण / Trade, Marketing and Consumer Protection, साइबर सुरक्षा / Cyber Safety and Security, कंप्यूटर सॉफ़्टवेयर / Computer Software, ऑपरेटिंग सिस्टम / Operating System, कंप्यूटर नेटवर्क / Computer Networks, वर्ड प्रोसेसिंग (MS Word) / Word Processing (MS Word), Terms & Abbreviations, प्रेज़ेंटेशन (MS PowerPoint) / Presentation Software (MS PowerPoint)
  15. Section 3 | Software Engineering: Fundamentals of Software Engineering, Software Requirements and Quality Assurance, Software Design, Estimation and Metrics, Software Maintenance and Configuration Management, Software Testing
  16. Section 3 | C Programming: C Fundamentals, Control Flow, Functions, Arrays & Pointers, Storage Classes, Structures & Enums, DMA, Macros, Scoping & File Handling
  17. Section 3 | OOPS, C++, JAVA, HTML: OOPS with C++, Java, HTML Basics, CSS
  18. Section 3 | Java: Java Fundamentals, Array, String, IO & References, OOPS with Java
  19. Section 3 | Python: Introduction to Python, Tokens, Variables, Data Types, I/O, Operators & Expressions, Control Flow & Looping, Collections & Strings, Functions & Modules, File Handling & Data Analysis
  20. Section 3 | Digital Electronics: Digital Systems & Boolean Basics, Logic Gates & Hardware, Boolean Expression, Boolean Minimization, Combinational Circuit, Sequential Circuits, Number System, Number Representation
  21. Section 3 | Multimedia: Multimedia
  22. BPSC Previous Year Papers:
  23. BPSC Full Mock Tests:
AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This educational video delivers a detailed lecture on the Shortest Seek Time First (SSTF) disk scheduling algorithm used in operating systems. The session begins by defining the algorithm's core heuristic: prioritizing requests that are physically closest to the current position of the disk read/write head. The instructor provides a step-by-step numerical example on the whiteboard to illustrate how the head moves to service requests, calculating the total seek time. The lecture then transitions into a critical analysis, presenting a slide that lists the specific advantages, such as decreased seek movements and increased throughput, alongside significant disadvantages like the overhead of calculating distances and the risk of starvation. The instructor uses diagrams to visually explain how the head can get trapped servicing nearby requests, leaving distant ones waiting indefinitely, and draws a parallel between SSTF and Shortest-Job-First (SJF) scheduling to reinforce the concept of starvation.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The instructor introduces the concept of SSTF scheduling, standing next to a slide titled "SSTF Scheduling." He explains the fundamental assumption: "It seems reasonable to service all the requests close to the current head position before moving the head far to service other REQUESTS." He writes a list of cylinder numbers on the board—47, 93, 3, 51, 29, 78—to create a scenario for the algorithm. He circles the number 40, indicating the current head position, and begins to explain that the request nearest to the disk arm will get executed first, effectively minimizing the seek time for the immediate next step. He notes that while this is an improvement over FCFS, it is not necessarily optimal, setting the stage for the example that follows.

  2. 2:00 – 5:00 02:00-05:00

    The instructor continues the worked example, demonstrating the decision-making process of the SSTF algorithm. He points to the slide text which states, "In the example, we can do better by moving the head from 53 to 37, even though the latter is not closest." He writes calculations on the board, such as "57 - 29 = 28," to show the distance between cylinders. He emphasizes the efficiency of the strategy by underlining the text "This strategy reduces the total head movement to 208 cylinders," contrasting it implicitly with the less efficient First-Come-First-Served (FCFS) algorithm mentioned in the text. He uses his hand to trace the path of the head on the slide, showing how the head moves to the nearest request before turning around.

  3. 5:00 – 10:00 05:00-10:00

    The presentation shifts to a slide listing the "Advantages" and "Disadvantages" of SSTF. The instructor marks checkmarks next to the advantages: "Seek movements decreases," "Average Response Time decreases," and "Throughput increases." He then focuses on the disadvantages, specifically highlighting "Overhead to calculate the closest request" and "Can cause Starvation for a request which is far from the current location of the header." To illustrate starvation, he draws a diagram showing the disk head moving back and forth between two close requests while a third request far away remains unserviced, visually demonstrating the "High variance of response time" mentioned on the slide. He points to the text "SSTF favours only some requests" to explain why the response time variance is high.

  4. 10:00 – 11:45 10:00-11:45

    The instructor concludes the lesson by drawing a direct comparison between SSTF and Shortest-Job-First (SJF) scheduling. He points to the text "SSTF scheduling is essentially a form of shortest-job-first (SJF) scheduling," explaining that just like SJF, SSTF may cause starvation of some requests. He circles the term "SSTF" in the disadvantage section to reinforce that while the algorithm is efficient for average response time, it is not optimal for all requests. He reiterates that the algorithm favors requests close to the current head position, leading to the potential neglect of distant requests, which is a critical consideration for system designers. He emphasizes that this is a form of "shortest-job-first" scheduling and that "like SJF scheduling, it may cause starvation of some requests."

The video effectively bridges the gap between theoretical definition and practical application of SSTF scheduling. It starts with the fundamental logic of minimizing seek time, moves to a concrete numerical demonstration to show how the algorithm operates, and concludes with a critical evaluation of its trade-offs. The instructor uses visual aids like board calculations and diagrams to clarify complex concepts like starvation, making the comparison to SJF scheduling clear and memorable for students. The progression from definition to example to pros/cons provides a complete understanding of the algorithm's role in operating systems, ensuring students grasp both the efficiency gains and the potential pitfalls of the strategy.

Loading lesson…