Disk Scheduling Algorithm - FCFS Scheduling

Scheduling is a key concept in computer multitasking, multiprocessing operating system and real-time operating system designs. The scheduler is concerned mainly with :
* CPU utilization - to keep the CPU as busy as possible.
* Throughput - number of process that complete their execution per time unit.
* Turnaround - total time between submission of a process and its completion.
* Waiting time - amount of time a process has been waiting in the ready queue.
* Response time - amount of time it takes from when a request was submitted until the first response is produced.
* Fairness - Equal CPU time to each thread.

Scheduling Algorithms :
CPU scheduling deals with the problem of deciding which of the processes in the ready queue is to be allocated the CPU. There are many different CPU scheduling algorithms.

- First Come First Served (FCFS): The disk controller processes the I/O requests in the order in which they arrive, thus moving backwards and forwards across the surface of the disk to get to the next requested location each time. Since no reordering of request takes place the head may move almost randomly across the surface of the disk. This policy aims to minimize response time with little regard for throughput.
Illustration shows total head movement of 640 cylinders.
queue = 98, 183, 37, 122, 14, 124, 65, 67
Head starts at 53.



The problem with this schedule is illustrated by the wide swing from 122 to 14 and then back to 124. If the requests for cylinders 37 and 14 could be serviced together, before or after the requests at 122 and 124, the total head movement could be decreased substantially, and performance could be improved.

First-Come-First-Served (FCFS) Scheduling

First-Come-First-Served algorithm is the simplest scheduling algorithm is the simplest scheduling algorithm. Processes are dispatched according to their arrival time on the ready queue. Being a non-preemptive discipline, once a process has a CPU, it runs to completion. The FCFS scheduling is fair in the formal sense or human sense of fairness but it is unfair in the sense that long jobs make short jobs wait and unimportant jobs make important jobs wait.
FCFS is more predictable than most of other schemes since it offers time. FCFS scheme is not useful in scheduling interactive users because it cannot guarantee good response time. The code for FCFS scheduling is simple to write and understand. One of the major drawback of this scheme is that the average time is often quite long.

CHARACTERISTICS :
o Non-preemptive.
o Ready queue is a FIFO queue.
o Jobs arriving are placed at the end of queue.
o Dispatcher selects first job in queue and this job runs to completion of CPU burst.

Advantages:
- Simple.
- Low overhead.
Disadvantages:
- Inappropriate for interactive systems.
- Large fluctuations in average turnaround time are possible.

Example :
Process Burst Time
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3.
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: (0 + 24 + 27)/3 = 17

Suppose that the processes arrive in the order P2 , P3 , P1.
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case.