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

Difference between SSTF and LOOK Disk Scheduling Algorithm

Shortest Seek Time First / SSTF

Shortest Seek Time First (SSTF) is an acronym for "shortest seek time first." The task request that is closest to the present position of the head or pointer is served by this algorithm. The direction of the head is crucial in estimating total head movement in this case. If there is a tie between requests, the request that confronts it in its current direction will be served by the head. The SSTF algorithm, unlike C-LOOK, is particularly efficient in terms of overall search time. Requests with least time demand are first processed in SSTF. As a result, every request's seek time is computed ahead of time in the queue, and requests are scheduled according to their predicted seek time. As a result, the request that is closest to the disc arm will be processed first. SSTF is a significant improvement over FCFS, since it reduces average response time and enhances system throughput.

It's a suitable batch processing system solution. It does not always assure justice because hunger is a possibility with this schedule. This policy is often unsuitable for interactive systems, as it leads to greater response time fluctuations.


  • When compared to FCFS, it decreases overall seek time.
  • It has a higher throughput and has a shorter average response time and waiting time.


  • There is a cost associated with locating the most recent request.
  • And queries that aren't close to the head may run out of CPU resources.
  • It offers a wide range of response and waiting times.
  • The algorithm is slowed by often changing the orientation of the head.

LOOK Disc Scheduling Algorithm

The LOOK algorithm is a better version of the SCAN algorithm. The head begins serving the first request on one side of the disc and advances to the opposite end of the disc, fulfilling all requests in between. Unlike SCAN, the head goes to the last request instead of the final track, and then the direction is adjusted. It does not, unlike SSTF, serve the task request that is closest to the current head or pointer location. It's identical to the SCAN disc scheduling method, except that instead of travelling to the end of the disc, the disc arm only goes to the last request to be handled in front of the head and then reverses course from there. As a result, the additional time caused by unneeded traversal to the disk's end is avoided.


  • When there are no requests to be served, it does not force the head to move until the end of the disc.
  • When compared to the SCAN Algorithm, it performs better.
  • It does not result in hunger.
  • It has a low reaction time and waiting time variance.


  • There is a cost associated with locating the final requests.
  • It also results in a significant delay for the cylinders that the head has just visited.

Main Difference

1.SCAN Even if there are no requests at the ends, the algorithm scans all of the cylinders on the drive from one end to the other.  Starting with the initial request at one end and ending with the final request at the other, the LOOK Algorithm examines all of the cylinders on the disc.  
2.The performance of the SSTF disc scheduling method is slow.The LOOK disc scheduling technique outperforms SSTF in terms of performance.
3.When compared to the LOOK disc scheduling technique, it decreases overall seek time.The overall seek time has been raised by the LOOK disc scheduling technique.
4.The average response time and waiting time for this method have both high variation.It has a low standard deviation in average waiting and response times.
5.The orientation of the head, as indicated in the picture above, is critical in breaking the tie between requests.When the LOOK disc scheduling algorithm fulfils the final request in one direction, the head's orientation is reversed.
6.There is a cost to identifying the nearest request in the SSTF disc scheduling method.There is a cost to locating end requests in this method.
7.The request that is distant from the head will be starved in the SSTF disc scheduling mechanism.The request that is far from the top of the queue will be starved. Any request is not starved by the LOOK disc scheduling mechanism.
8.Request handling is not as excellent in the SSTF disc scheduling method as it is in the LOOK algorithm.The LOOK disc scheduling technique is more efficient than SSTF at handling requests.