Shortest Seek Time First Scheduling

Duration: 12 min

This video lesson is available to enrolled students.

Return to /learn/UP-LT-GRADE-ASSISTANT-TEACHER-2025/cs-06-operating-system/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 Course: About the Course
  2. CS | 01 | Digital Logic and Circuits and Discrete Mathematical Structures: Digital Systems & Boolean Basics, Number System, Number Representation, Logic Gates & Hardware, Boolean Expression, Boolean Minimization, Combinational Circuit, Sequential Circuits, Set Theory, Relations, Functions, Graph Theory, Group Theory, Propositional and Predicate Logic, Permutation and Combination
  3. CS | 02 | Computer Organization and Architecture: Basics of COA, Cache Memory Organization, Input Output Organisation, Pipelining, Instr Formats & Modes, Control Unit Design, Multiprocessors
  4. CS | 03 | Data Structures and Algorithm: Introduction to DS, Array, Stack, Queue, Linked List, Tree, Graphs, Hashing, Algorithm Analysis, Time Complexity Analysis, Sorting Algorithms, Greedy Algorithms, Dynamic Programming, Minimum Spanning Trees, Shortest Path Algos, Complexity Theory, Advanced Algorithms
  5. CS | 04 | Problem Solving through C Programming: C Fundamentals, Control Flow, Functions, Arrays & Pointers, Storage Classes, Structures & Enums, DMA, Macros, Scoping & File Handling
  6. CS | 05 | Object Oriented Techniques: Object Oriented Technology, Java
  7. CS | 06 | Operating System: Introduction to OS, Process Management, CPU Scheduling, Process Synchronization, Threads & Process Creation, Deadlock, Memory Management, Virtual Memory, Disc Scheduling, File Management, Windows OS, Unix OS
  8. CS | 07 | Database Management Systems: Basics of DBMS, ER Diagram, Relational Model & Functional Dependencies, Keys & Integrity Constraints, Normalization (1NF - BCNF), Decomposition Properties & 4NF, Relational Algebra, SQL, Relational Calculus, Transaction Management, Concurrency Control, Database Recovery, Database Security & Authorization, Query Processing & Optimization, File Organization & Indexing
  9. CS | 08 | Computer Networks: Introduction to CN, Data Communication, DLL: Access Control, DLL: Flow Control, DLL: Error Control, DLL: Framing, Data Link Layer - Ethernet, Net Layer: IPv4 & Proto, Net Layer: IP Addressing, Net Layer:Routing Protocol, Transport Layer Services, TL: Congestion & UDP, Application Layer, Hardware basics
  10. CS | 09 | Software Engineering: Fundamentals of Software Engineering, Software Requirements and Quality Assurance, Software Design, Estimation and Metrics, Software Testing, Software Maintenance and Configuration Management
  11. CS | 10 | Internet Technology, Web Design and Web Technology: Internet Fundamentals, Web Design & Development, Protocols & Services, Web Publishing & Hosting, Server Side & Security, Emerging Technologies, JavaScript-Basics, HTML Basics
  12. CS | 11 | System Analysis And Design: Fundamentals of Systems and Information Systems, SDLC and Development Approaches, Software Documentation and Software Requirements Specification (SRS), SDD and Quality Assurance, Security, Risk Management & Maintenance
  13. CS | 12 | Information Security and Cyber Laws: Cyber Threats and Malware Attacks, Role of Internet and Web Services, Security in Mobile and Wireless Computing, Electronic Data Interchange and E-Commerce, Physical Security and Cryptography Basics, Intrusion Detection Systems (IDS), Cyber Safety and Security
  14. CS | 13 | Computer Graphics: Basics of Computer Graphics, 2-D Geometrical Transforms and Viewing, 3-D Object Representation, Geometric Transformations and Viewing
  15. GS Paper | 01 | Indian and World Geography: Geography
  16. GS Paper | 02 | History: History
  17. GS Paper | 03 | Indian Economy & Social Development: Indian Economy
  18. GS Paper | 04 | General Science: Biology, Chemistry, वैज्ञानिक उपलब्धियाँ और अनुसंधान / Scientific Achievements and Research
  19. GS Paper | 05 | Elementary Maths: संख्या पद्धति / Number System, Divisibility Rules, महत्तम समापवर्तक और लघुत्तम समापवर्त्य / HCF and LCM, प्रतिशत / Percentage, लाभ, हानि और बट्टा / Profit, Loss and Discount, साधारण और चक्रवृद्धि ब्याज / Simple and Compound Interest, अनुपात और समानुपात / Ratio and Proportion, औसत / Average, समय और कार्य / Time and Work, समय, चाल और दूरी / Time, Speed and Distance, Algebra, घन क्षेत्रमिति / Solid Mensuration, आँकड़ा निर्वचन / Data Interpretation, Arithmetic, Statistics, Probability, Sequence and Series (Series), मिश्रण और एलिगेशन / Mixture and Alligation, Calculus, Trigonometry, Vectors, संख्या पद्धति और संख्या सिद्धांत / Number Systems and Number Theory, यूक्लिडीय ज्यामिति और क्षेत्रमिति / Euclidean Geometry and Mensuration, सांख्यिकी और प्रायिकता / Statistics and Probability, निर्देशांक ज्यामिति, सदिश और त्रिविमीय ज्यामिति / Coordinate Geometry, Vectors and 3D Geometry, बहुपद, समीकरण और सम्मिश्र संख्याएँ / Polynomials, Equations and Complex Numbers, समतल क्षेत्रमिति / Plane Mensuration, सरलीकरण, घातांक और करणी / Simplification, Indices and Surds, समुच्चय, संबंध, फलन और विविक्त गणित / Sets, Relations, Functions and Discrete Mathematics
  20. GS Paper | 06 | Indian Polity and Governance: Polity
  21. GS Paper | 07 | Current Events: समसामयिकी / Current Affairs
  22. UPLT Live Classes Notes: UPLT Live Classes
  23. UPLT Full Mock Tests Tier 1:
  24. UPLT Full Mock Tests Tier 2:
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…