Question

Use the following list of cylinder requests in Exercises 54-56. They are listed in the order in which they were received. \(40,12,22,66,67,33,80\) List the order in which these requests are handled if the SSTF algorithm is used. Assume that the disk is positioned at cylinder 50 .

Short answer

The requests are handled in the order: 40, 33, 22, 12, 66, 67, 80.

Step-by-step solution

Understanding SSTF Scheduling

SSTF stands for Shortest Seek Time First. The algorithm handles disk requests by selecting the request that is closest in terms of the cylinder number to the current position of the disk head. This minimizes the seek time at each step.

Initial Position and Requests

The current position of the disk head is at cylinder 50. The pending requests are: 40, 12, 22, 66, 67, 33, and 80.

Calculate Distances from Initial Position

From cylinder 50, calculate the distance to each request: - |50 - 40| = 10 - |50 - 12| = 38 - |50 - 22| = 28 - |50 - 66| = 16 - |50 - 67| = 17 - |50 - 33| = 17 - |50 - 80| = 30 The closest request to 50 is 40.

First Move to Closest Request

The disk head moves from 50 to 40. Now the current position is at cylinder 40, and the remaining requests are 12, 22, 66, 67, 33, 80.

Calculate Distances from New Position

From cylinder 40, calculate the distance to each remaining request: - |40 - 12| = 28 - |40 - 22| = 18 - |40 - 66| = 26 - |40 - 67| = 27 - |40 - 33| = 7 - |40 - 80| = 40 The closest request to 40 is 33.

Second Move to Closest Request

The disk head moves from 40 to 33. Now the current position is at cylinder 33, and the remaining requests are 12, 22, 66, 67, 80.

Calculate Distances for Next Position

From cylinder 33, calculate the distance to each remaining request: - |33 - 12| = 21 - |33 - 22| = 11 - |33 - 66| = 33 - |33 - 67| = 34 - |33 - 80| = 47 The closest request to 33 is 22.

Third Move to Closest Request

The disk head moves from 33 to 22. The current position is now at cylinder 22, and the remaining requests are 12, 66, 67, 80.

Calculate Distances Again

From cylinder 22, calculate the distance to each remaining request: - |22 - 12| = 10 - |22 - 66| = 44 - |22 - 67| = 45 - |22 - 80| = 58 The closest request to 22 is 12.

Fourth Move to Closest Request

The disk head moves from 22 to 12. The current position is now at cylinder 12, and the remaining requests are 66, 67, 80.

Calculate Distances Further

From cylinder 12, calculate the distance to the remaining requests: - |12 - 66| = 54 - |12 - 67| = 55 - |12 - 80| = 68 The closest request to 12 is 66.

Fifth Move to Closest Request

The disk head moves from 12 to 66. The current position is now at cylinder 66, and the remaining requests are 67, 80.

Calculate Final Distances

From cylinder 66, calculate the distance to each remaining request: - |66 - 67| = 1 - |66 - 80| = 14 The closest request to 66 is 67.

Sixth Move to Closest Request

The disk head moves from 66 to 67. The current position is now at cylinder 67, and the last request is 80.

Final Move to Last Request

Finally, the disk head moves from 67 to 80, completing all the requests.

Key concepts

Disk Scheduling Algorithms

Disk scheduling algorithms are crucial in determining the efficiency and performance of disk usage in operating systems. These algorithms manage the order in which read and write requests are processed on the disk. The primary goal is to minimize the total time taken to service these requests, also known as seek time. Various algorithms achieve this with different approaches.

Here are some commonly used disk scheduling algorithms:

• **First-Come, First-Served (FCFS)**: Processes requests in the order they come. It's simple but can be inefficient.
• **Shortest Seek Time First (SSTF)**: Prioritizes requests closest to the current head position. This improves response time but can lead to request starvation.
• **Elevator (SCAN) Algorithm**: Moves the head in a single direction to the end, then reverses direction, resembling an elevator's movement.
• **C-SCAN**: Similar to SCAN but only returns to the start in one direction, improving wait times for some requests.

Each algorithm has its benefits and trade-offs, focusing on reducing seek time and improving overall disk efficiency.

Seek Time Optimization

Seek time optimization is essential in ensuring that disk operations are performed quickly and efficiently. Seek time refers to the time taken for the disk's read/write head to move to the desired track or cylinder. Minimizing seek time directly improves a system's performance by reducing the delay in processing requests.

Several techniques are employed to optimize seek time:

• **Efficient Scheduling**: Algorithms like SSTF and LOOK focus on minimizing the movement of the disk arm by selecting the nearest request.
• **Cylindrical Layout**: Data is organized on disks to minimize head movement during sequential reads and writes.
• **Request Batching**: Grouping similar requests can reduce redundant head movements.

By carefully managing seek time, systems can achieve faster data access and improved throughput.

Cylinder Requests Management

Cylinder requests management is about handling various read and write requests to the disk's cylinders efficiently. A cylinder is essentially the same track on all platters of a hard drive, and managing requests effectively means determining the optimal sequence to service these requests.

Here are some strategies for better cylinder requests management:

• **Request Queuing**: Requests are lined up based on their arrival or priority, setting the stage for sequential handling.
• **Prioritization**: Certain algorithms prioritize requests based on proximity to reduce the workload on the disk head.
• **Dynamic Strategies**: Some systems adapt the request handling pattern based on current load conditions for optimal performance.

Effective cylinder requests management improves drive efficiency and reduces overall latency.

Shortest Seek Time First Algorithm

The Shortest Seek Time First (SSTF) algorithm is one of the many strategies for disk scheduling that minimize the time it takes to access a particular disk cylinder. This algorithm chooses the pending request closest to the current disk head position, effectively reducing the seek time, which is the main delay factor in disk operations.

Here's a simple breakdown of how SSTF works:

• **Calculate Distances**: For each incoming request, the distance from the current head position is calculated.
• **Select Closest Request**: The request with the shortest distance is serviced next.
• **Move Disk Head**: The head moves to this new location, and the process repeats for the remaining requests.

Though highly effective in improving responsiveness, SSTF can potentially lead to request starvation if new requests keep coming between the current head and an older request. However, its ability to optimize seek time makes it a popular choice in disk scheduling.