Question

operating systems question:

a)What scheduling algorithm(s) provide a bound on response time?
b)How is this bound implemented? (Explain)

Explain each with a few sentences

Answer 1

a)

• First Come First Serve scheduling algorithm
• Shortest Job Next sceduling alogorithm
• Priority Scheduling
• Round Robin scheduling
• Multilevel Queues scheduling

b)

• First come First serve scheduling algorithm

With this method,process which request first,that process will get the cpu first.The execution of the FCFS policy is easily managed with a FIFO queue. As a process enters the ready queue, its Process Control Block is linked with the tail of the queue. When the CPU gets free, it is assigned to the process at the head or start of the queue.

Example :-

Consider the following set of processes/jobs which arrive at time 0, with the length of the CPU burst that is given in milliseconds:

process list	Burst-Time
P1	24
P2	3
P3	3

When the processes arrive in the order - P1, P2, P3 and is served using FCFS method, you get the outcome as given in the below mentioned Gantt chart:

P1

P2

P3

0 24 27 30

• Shortest Job Next sceduling alogorithm

A diverse approach to CPU scheduling is the technique of shortest-job-first (SJF) scheduling algorithm which links with each process the length of the process's next CPU burst. If the CPU is available, it is assigned to the process that has the minimum next CPU burst. If the subsequent CPU bursts of two processes become the same, then FCFS scheduling is used to break the tie.

Example :-

An example of SJF scheduling, with the given set of processes below, and the length of the CPU burst in milliseconds:

Process List	Burst time
P1	6
P2	8
P3	7
P4	3

P4

P1

P3

P2

0 3 9 16 24

• Priority Scheduling

  A priority is related and assigned with each process, and the CPU gets assigned to the process with the maximum priority. Equal priority processes get scheduled using FCFS method. An SJF algorithm is purely a priority algorithm wherein the priority (P) is the opposite of the (predicted) subsequent CPU burst. The better the CPU burst, the lower the priority is and vice versa.

• Round Robin scheduling

The round-robin (RR) scheduling technique is intended mainly for time-sharing systems. in this technique,A small unit of time which is termed as a time quantum or time slice has to be defined. A 'time quantum' is usually from 10 to 100 milliseconds. The ready queue gets treated with a circular queue. The CPU scheduler goes about the ready queue, allocating the CPU with each process for the time interval which is at least 1-time quantum.

• Multilevel Queues scheduling

Another form of scheduling technique has been designed for situations where processes are simply classified into different groups. A multi-level queue scheduling technique partitions or divides the ready queue into many separate queues. The processes get permanently assigned to one queue, usually based on some property of the process, such as the size of the memory, process priority and/or type of process. Each queue got its scheduling algorithm which works at the multilevel form.