Elevator algorithm (scan)

Definition

The elevator algorithm, also known as the SCAN algorithm, is a disk scheduling method that optimizes the order of read and write requests by moving the disk arm in a linear path, servicing requests in one direction until it reaches the end of the disk, and then reversing direction. This method resembles an elevator that moves up and down, hence the name, ensuring that requests are handled efficiently while minimizing seek time and increasing throughput.

5 Must Know Facts For Your Next Test

1. The elevator algorithm effectively reduces average seek time compared to FCFS by servicing requests in a more organized manner.
2. One potential drawback is that requests located at the extreme ends of the disk may experience longer wait times if they are not aligned with the direction of travel.
3. The SCAN algorithm is particularly effective for workloads with predictable access patterns since it continuously services requests in one direction before reversing.
4. By handling requests in a sweeping motion, this algorithm can improve throughput as it minimizes back-and-forth movements of the disk arm.
5. Variations of the elevator algorithm include C-SCAN and LOOK, which attempt to further optimize performance by modifying how the arm travels across the disk.

Review Questions

• How does the elevator algorithm improve upon simpler disk scheduling methods like FCFS?
  • The elevator algorithm improves upon FCFS by servicing requests in a systematic way rather than in strict arrival order. This allows the disk arm to reduce its seek time by minimizing unnecessary movements. By traveling in one direction and then reversing, it optimizes the handling of multiple requests, leading to better overall performance.
• Discuss some potential limitations or challenges associated with using the elevator algorithm for disk scheduling.
  • One challenge of using the elevator algorithm is that it can lead to increased wait times for requests located at the far ends of the disk if they happen to be serviced last. Additionally, if there are frequent incoming requests, they may be queued up behind existing ones, which could lead to delays. Balancing fairness and efficiency becomes important when implementing this algorithm.
• Evaluate how variations like C-SCAN and LOOK enhance or modify the basic principles of the elevator algorithm.
  • C-SCAN enhances the basic principles of the elevator algorithm by treating the disk as circular, allowing the arm to return to the beginning after reaching one end. This ensures that all areas of the disk are serviced more evenly over time. LOOK modifies it by only going as far as there are requests in either direction, which can further minimize unnecessary movements and improve efficiency by reducing wait times for those requests not at extreme ends.

Related terms

FCFS (First-Come, First-Served): A simple disk scheduling algorithm that services requests in the order they arrive, without any reordering.

SSTF (Shortest Seek Time First): A disk scheduling method that selects the request closest to the current head position, aiming to reduce seek time.

C-SCAN (Circular SCAN): An improvement of the SCAN algorithm that treats the disk as circular, allowing the arm to return to the beginning after reaching the end, thus reducing wait time for requests at the start.