Imagine you are standing in a queue at a grocery store. The cashier serves the customers one by one in the order they arrived. The first person who comes to the queue gets served first, the second person goes next, and so on.
Imagine you are standing in a queue at a grocery store. The cashier serves the customers one by one in the order they arrived. The first person who comes to the queue gets served first, the second person goes next, and so on.
This is exactly how the First Come, First Serve (FCFS) scheduling algorithm works in an operating system.
| Process | Arrival Time | Burst Time |
|---|---|---|
| P1 | 0 ms | 5 ms |
| P2 | 1 ms | 3 ms |
| P3 | 2 ms | 8 ms |
Execution Order: P1 → P2 → P3 (since P1 arrived first, it gets executed first and then other processes).
Imagine you are standing in the middle of a long hallway with multiple rooms on both sides. Your job is to deliver packages to these rooms, but you can choose the order in which you deliver them. Naturally, you would deliver to the closest room first to minimize walking distance, right?
In SSTF, the disk head moves to the closest track in the queue, minimizing seek time. This improves overall system performance but can lead to starvation of some requests.
Let’s say the disk head is at position 50, and we have requests at these positions:
Requests: 20, 45, 65, 10, 90
Execution Order (SSTF chooses the closest first):
Instead of blindly serving in the arrival order, SSTF intelligently reduces movement to speed things up!
Imagine you are standing in a long hallway with multiple rooms on both sides. Your job is to deliver packages to these rooms, but you can only move in one direction at a time. You start at one end, deliver packages to the rooms, and then reverse direction to deliver the remaining packages.
The SCAN algorithm, also known as the elevator algorithm, moves the disk head back and forth across the disk surface, servicing requests in each direction.
Let’s say the disk head is at position 50, and we have requests at these positions:
Requests: 20, 45, 65, 10, 90
Execution Order (SCAN moves in one direction, then reverses):
SCAN works like an elevator, moving in one direction until the last request, then reversing to serve remaining requests in the opposite direction!
Imagine you are a bus driver following a fixed route. You start picking up passengers from one end of the city and move towards the other end, stopping at all bus stops along the way. Once you reach the last stop, instead of turning back and picking up passengers on the way back, you go straight to the first stop and start over.
C-SCAN (Circular SCAN) is a variant of the SCAN algorithm that provides more uniform waiting time by treating the disk as a circular list.
Let’s say the disk head is at 50, and requests are at these positions:
Requests: 20, 35, 75, 85, 95
Execution Order (C-SCAN moves one way, resets, and continues):
Instead of going back and forth like an elevator (SCAN), C-SCAN always moves in one direction and jumps back to start again.