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Difference Between Multi-programming and Multitasking Difference between C-LOOK and C-SCAN Difference between Rotational Latency and Disk Assess Time Trap vs Interrupt Difference between C-SCAN and SSTF Difference between SCAN and FCFS Difference between Seek Time and Disk Access Time Difference between SSTF and LOOK Difference between Process and Program in the Operating System Difference between Protection and Security in Operating System

How To

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What is Kernel and Types of Kernel What is DOS Operating System What is Thread and Types of Thread What is Process Scheduler and Process Queue What is Context Switching What is CPU Scheduling What is Producer-Consumer Problem What is Semaphore in Operating System Monitors in Operating System What is Deadlock What is Paging and Segmentation What is Demand Paging What is Virtual Memory What is a Long term Scheduler What is Page Replacement in Operating System What is BSR Mode What is Convoy Effect What is Job Sequencing in Operating System Why is it critical for the Scheduler to distinguish between I/O-bound and CPU-bound programs Why is there a Need for an Operating System


Process Management Process State Scheduling Algorithm FCFS (First-come-First-Serve) Scheduling SJF (Shortest Job First) Scheduling Round-Robin CPU Scheduling Priority Based Scheduling HRRN (Highest Response Ratio Next) Scheduling Process Synchronization Lock Variable Mechanism TSL Mechanism Turn Variable Mechanism Interested Variable Mechanism Deadlock Avoidance Strategies for Handling Deadlock Deadlock Prevention Deadlock Detection and Recovery Resource Allocation Graph Banker’s Algorithm in Operating System Fixed Partitioning and Dynamic Partitioning Partitioning Algorithms Disk Scheduling Algorithms FCFS and SSTF Disk Scheduling Algorithm SCAN and C-SCAN Disk Scheduling Algorithm Look and C-Look Disk Scheduling Algorithm File in Operating System File Access Methods in Operating System File Allocation Method Directory Structure in Operating System N-Step-SCAN Disk Scheduling Feedback Queue in Operating System Contiguous Memory Allocation in Operating System Real-time Operating System Starvation in Operating System Thrashing in Operating System 5 Goals of Operating System Advantages of Operating System Advantages of UNIX Operating System Bit Vector in Operating System Booting Process in Operating System Can a Computer Run Without the Operating System Dining Philosophers Problem in Operating System Free Space Management in Operating System Inter Process Communication in Operating System Swapping in Operating System Memory Management in Operating System Multiprogramming Operating System Multitasking Operating Systems Multi-user Operating Systems Non-Contiguous Memory Allocation in Operating System Page Table in Operating System Process Scheduling in Operating System Segmentation in Operating System Simple Structure in Operating System Single-User Operating System Two Phase Locking Protocol Advantages and Disadvantages of Operating System Arithmetic operations in binary number system Assemblers in the operating system Bakery Algorithm in Operating System Benefits of Ubuntu Operating System CPU Scheduling Criteria in Operating System Critical Section in Operating System Device Management in Operating System Linux Scheduler in Operating System Long Term Scheduler in Operating System Mutex in Operating System Operating System Failure Peterson's Solution in Operating System Privileged and Non-Privileged Instructions in Operating System Swapping in Operating System Types of Operating System Zombie and Orphan Process in Operating System 62-bit operating system Advantages and Disadvantages of Batch Operating System Boot Block and Bad Block in Operating System Contiguous and Non - Contiguous Memory Allocation in Operating System Control and Distribution Systems in Operations Management Control Program in Operating System Convergent Technologies in Operating System Convoy Effect in Operating System Copy Operating Systems to SSD Core Components of Operating System Core of UNIX Operating System Correct Value to return to the Operating System Corrupted Operating System Cos is Smart Card Operating System Cosmos Operating Systems Examples Generation of Operating System Hardware Solution in Operating System Process Control Block in Operating System Function of Kernel in Operating System Operating System Layers History of Debian Operating Systems Branches and Architecture of Debian Operating Systems Features and Packages of Debian Operating Systems Installation of Operating System on a New PC Organizational Structure and Development in Debian Operating Systems User Interface in Operating System Types Of Memory in OS Operating System in Nokia Multilevel Paging in OS Memory Mapping Techniques in OS Memory Layout of a Process in Operating System Hardware Protection in Operating System Functions of File Management in Operating System Core of Linux Operating System Cache Replacement Policy in Operating System Cache Line and Cache Size in Operating System What is Memory Mapping? Difference Between Network Operating System And Distributed Operating System What is the difference between a Hard link and a Soft Link? Principles of Preemptive Scheduling Process Scheduling Algorithms What is NOS? What is the Interrupt I/O Process? What is Time Sharing OS What is process termination? What is Time-Sharing Operating System What is Batch File File system manipulation What is Message-passing Technique in OS Logical Clock in Distributed System

FCFS (First-Come-First-Serve) Disk Scheduling Algorithm

FCFS Disk Scheduling Algorithm

FCFS (First-Come-First-Serve) is the easiest disk scheduling algorithm among all the scheduling algorithms. In the FCFS disk scheduling algorithm, each input/output request is served in the order in which the requests arrive. In this algorithm, starvation does not occur because FCFS address each request.

Advantages of FCFS Disk scheduling Algorithm

The advantages of FCFS disk scheduling algorithm are:

  1. In FCFS disk scheduling, there is no indefinite delay.
  2. There is no starvation in FCFS disk scheduling because each request gets a fair chance.

Disadvantages of FCFS Disk Scheduling Algorithm

The disadvantages of FCFS disk scheduling algorithm are:

  1. FCFS scheduling is not offered as the best service.
  2. In FCFS, scheduling disk time is not optimized.

Example of FCFS Disk Scheduling Algorithm

Suppose a disk contains 200 tracks (0-199) and the request queue contains track no: 93, 176, 42, 148, 27, 14,180. The current position of the read/write head is 55. Now we have to calculate the total number of track movements of read/write head using FCFS scheduling.


As mentioned in the following example, the disk contains 200 tracks, so we take a track line between 0 to 199.

The current position of the read/write head is 55. So, we start from 55, then move read/write head in the FCFS order. When all the requests are addressed, then we calculate a total number of cylinders moved by the head.

FCFS Disk Scheduling Algorithm
Figure: FCFS Disk Scheduling

Total Number of cylinders moved by the head = (176-55) + (176-42) + (148-42) + (148-14) + (180-14)

                                                                     = 121+134+106+134+166


SSTF (Shortest Seek Time First) Disk Scheduling Algorithm

SSTF is another type of scheduling algorithm. In this type of disk scheduling, the job which has less seek time will be executed first. So, in SSTF (shortest seek time first) scheduling, we have to calculate the seek time first. and after calculating the seek time, each request will be served on the basis of seek time. The request which is close to the disk arm will be first executed. There are some drawbacks in FCFS. To overcome the limitations that arise in the FCFS. SSTF scheduling is implemented.

Advantages of SSTF Disk Scheduling

The advantages of SSTF disk scheduling are:

  1. In SSTF disk scheduling, the average response time is decreased.
  2. Increased throughput.

Disadvantages of SSTF Disk Scheduling

The disadvantages of SSTF disk scheduling are:

  1. In SSTF, there may be a chance of starvation.
  2. SSTF is not an optimal algorithm.
  3. There are chances of overhead in SSTF disk scheduling because, in this algorithm, we have to calculate the seek time in advanced.
  4. The speed of this algorithm can be decreased because direction could be switched frequently.

Example of SSTF Disk Scheduling

Consider a disk that contains 200 tracks (0-199). The request queue includes track number 82, 170, 43, 140, 24, 16, 190, respectively. The current position of the read/write head is 50.


Before solving the above example, we have to know about the seek time.

Seek Time: -Seek time is the time required to move the desired track.

To find the seek time, we can use this simple formula.

                                            seek time = Destination – Source


                                             = Source - Destination

 Now, we can solve the given example.

As mentioned in the following example, disk contains 200 tracks. So, we will take a track line between 0 to 199. The current position of the read/write head is 50. So, we start at 50.

SSTF Disk Scheduling Algorithm
Figure: SSTF Disk Scheduling

We can see in the following figure that the current or initial position of read/write head is 50. Now for further movement of read/write head, we calculate the seek time. 

Total Number of cylinders moved by the head = (50-16) + (190-16)

                                                                     = 208