Operating System Tutorial

What is Operating System Evolution of Operating System Types of Operating System Functions of Operating System What is Kernel and Types of Kernel Operating System Properties Operating System Services Components of Operating System Needs of the Operating System Linux Operating System Unix Operating System Ubuntu Operating System What is DOS Operating System Difference Between Multi-programming and Multitasking What is Thread and Types of Thread Process Management Process State What is Process Scheduler and Process Queue What is Context Switching What is CPU Scheduling 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 What is Producer-Consumer Problem What is Semaphore in Operating System Monitors in Operating System What is Deadlock 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 What is Paging and Segmentation What is Demand Paging What is Virtual Memory 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 Difference between C-LOOK and C-SCAN Difference between Rotational Latency and Disk Assess Time Trap vs Interrupt How to implement Monitors using Semaphores N-Step-SCAN Disk Scheduling Why is it critical for the Scheduler to distinguish between I/O-bound and CPU-bound programs Difference between C-SCAN and SSTF Difference between SCAN and FCFS Difference between Seek Time and Disk Access Time Difference between SSTF and LOOK

Look Disk Scheduling

Look Disk Scheduling

Look disk scheduling is another type of disk scheduling algorithm. Look scheduling is an enhanced version of SCAN disk scheduling. Look disk scheduling is the same as SCAN disk scheduling, but in this scheduling, instead of going till the last track, we go till the last request and then change the direction.

Advantages of Look Disk Scheduling

The advantages of look disk scheduling are:

  1. In Look disk scheduling, there is no starvation.
  2. Look disk scheduling offers low variance in waiting time and response time.
  3. Look disk scheduling offers better performance as compared to the SCAN disk scheduling.
  4. In look disk scheduling, there is no requirement of disk head to move till the end to the disk when we do not have any request to be serviced.

 Disadvantages of Look Disk Scheduling

The disadvantages of Look disk scheduling are:

  1. In look disk scheduling, there is more overhead to find the end request.
  2. Look disk scheduling is not used in case of more load.

Example of Look Disk Scheduling

Consider a disk contains 200 tracks (0-100). The request queue includes track number 82, 170, 43, 140, 24, 16, 190, respectively. The current position of the read/write head is 50. The direction is towards the larger value. Calculate the total number of cylinders moved by head using look disk scheduling.

Solution:

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

The current position of the read/write head is 50, So we start at 50, then we move the read/write head. (the disk head is moved towards the larger value as mentioned in the given example). When all the requests are addressed, then we calculate the total number of cylinders moved by the head.

Look Disk Scheduling

                                                               Figure: Look Disk Scheduling

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

                                                                             = 314

C-Look Disk Scheduling

C-look means circular-look. It takes the advantages of both the disk scheduling C-SCAN, and Look disk scheduling. In C-look scheduling, the disk arm moves and service each request till the head reaches its highest request, and after that, the disk arm jumps to the lowest cylinder without servicing any request, and the disk arm moves further and service those requests which are remaining.

Advantages of C-Look Disk Scheduling

The advantages of C-look disk scheduling are:

  1. There is no starvation in C-look disk scheduling.
  2. The performance of the C-Look scheduling is better than Look disk scheduling.
  3. C-look disk scheduling offers low variance in waiting time and response time.

Disadvantages of C-Look Disk Scheduling

The disadvantages of C-Look disk scheduling are:

  1. In C-Look disk scheduling there may be more overhead to determine the end request.
  2. There is more overhead in calculations.

Example of C-Look Disk Scheduling

Consider a disk containing 200 tracks (0-100). The request queue contains the track number 93, 176, 42, 148, 27, 14,183 respectively. The current position of the R/W head is 55. The direction is towards the larger value. Calculate the total number of cylinders moved by head using look disk scheduling.

Solution:

As mentioned in the following example, 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 at 55, then we move the read/write head (the disk head is moved towards the larger value as mentioned in the given example). When all the requests are addressed, then we calculate the total number of cylinders moved by the head.

Look Disk Scheduling

                                                                   Figure: C-Look Disk Scheduling  

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

                                                                    = 125 + 166 + 28

                                                                             = 319



ADVERTISEMENT
ADVERTISEMENT