Two scheduling strategies for an operating system scheduler are first come first

Question

Two scheduling strategies for an operating system scheduler are first come first serve (FCFS) and fixed priority pre-emptive scheduling (FPPS). Since queues operate on a first come first serve basis, FCFS is implemented using a queue. Similarly, FPPS is implemented using a priority queue.

a) What are differences in how the jobs are managed between FCFS and FPPS?

b) What is are the advantages of FCFS over FPPS and vice versa?

c) What potential problems do you see happening if you were using an operating system with an FCFS scheduler?

d) What potential problems do you see happening if you were using an operating system with an FPPS scheduler?

Explanation / Answer

a)

First Come First Serve(FCFS) :

It uses queue to serve processes. The process which comes first is given the CPU first and is processed. There is no preemption in this case.

Fixed Priority Pre-emptive Scheduling(FPPS) :

In this scheduling, fixed priority is assigned to each process and are executed accordingly. Process with higher priority are executed first . In this, lower priority process can be preempted by higher priority process.

b)

FPPS can suffer from starvation if higher priority process keeps on coming and pre-empting the lower priority process.

There is no starvation problem in FCFS.

FCFS suffers from convoy effect i.e. short process keeps on waiting for long process to release the CPU . There is no such effect in FPPS.

c)

In FCFS, if long process keeps on holding CPU resulting in short process to wait for long time , throughput will be low.

Also, waiting time and turnaround time can be low .

d)

FPPS suffers from starvation problem where a lower priority process keeps on waiting for CPU while higher priority process keeps queuing for CPU.

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