Operating System Questions Long
In operating systems, disk scheduling algorithms are used to determine the order in which disk I/O requests are serviced. These algorithms aim to optimize the disk access time and improve overall system performance. There are several types of disk scheduling algorithms, each with its own advantages and disadvantages. The most commonly used disk scheduling algorithms are:
1. First-Come, First-Served (FCFS):
- This is the simplest disk scheduling algorithm where the requests are serviced in the order they arrive.
- It suffers from the "convoy effect" where a large request at the beginning of the queue can cause subsequent smaller requests to wait for a long time.
2. Shortest Seek Time First (SSTF):
- This algorithm selects the request that requires the least movement of the disk arm from its current position.
- It minimizes the average seek time and reduces the convoy effect.
- However, it may lead to starvation of requests located far from the current position of the disk arm.
3. SCAN:
- Also known as the elevator algorithm, SCAN moves the disk arm in one direction, servicing requests along the way until it reaches the end of the disk.
- Then, it changes direction and services the remaining requests in the opposite direction.
- This algorithm provides a fair distribution of service and avoids starvation.
- However, it may cause delays for requests located at the extreme ends of the disk.
4. Circular SCAN (C-SCAN):
- C-SCAN is an improved version of the SCAN algorithm.
- It works similar to SCAN but instead of moving the disk arm to the beginning of the disk after reaching the end, it moves it to the other end directly.
- This eliminates the delays caused by SCAN for requests at the extreme ends of the disk.
5. LOOK:
- LOOK is a variant of SCAN where the disk arm only goes as far as the last request in the current direction.
- It avoids servicing requests located at the extreme ends of the disk, reducing the average seek time.
- However, it may still cause starvation for requests located far from the current position of the disk arm.
6. Circular LOOK (C-LOOK):
- C-LOOK is an improved version of the LOOK algorithm.
- It works similar to LOOK but instead of moving the disk arm to the beginning of the disk after reaching the last request, it moves it to the last request in the opposite direction directly.
- This eliminates the delays caused by LOOK for requests at the extreme ends of the disk.
These are some of the commonly used disk scheduling algorithms in operating systems. The choice of algorithm depends on the specific requirements of the system and the workload characteristics. Each algorithm has its own trade-offs in terms of seek time, fairness, and starvation prevention.