Consider the following set of processes, with the length of the CPU-burst time given in milliseconds:

Question

Q. Consider the following set of processes, with the length of the CPU-burst time given in milliseconds:

Process Burst Time

P1 66

P2 33

P3 57

P4 20

P5 25

The processes are assumed to have arrived in the order P1, P2, P3, P4, P5, all at time 0.

a) Draw three Gantt charts illustrating the execution of these processes using FCFS, SJF, and RR (quantum = 20) scheduling.

b) What is the waiting time of each process for each of the scheduling algorithms in part a?

c) Which of the schedules in part a results in the minimal average waiting time (over all processes)?

Answer 1

First Come First Served (FCFS)

| Process | Burst Time | Start Time |

+---------+------------+------------+

| P1 | 66 | 0 |

| P2 | 33 | 66 |

| P3 | 57 | 99 |

| P4 | 20 | 156 |

| P5 | 25 | 176 |

+---------+------------+------------+

1 1 1 1 1 2

2 4 6 8 0 2 4 6 8 0

0 0 0 0 0 0 0 0 0 0 0

+----+----+----+----+----+----+----+----+----+----+

| P1 | P2 | P3 | P4 | P5 |

Smallest Job First (SJF)

| Process | Burst Time | Start Time |

+---------+------------+------------+

| P4 | 20 | 0 |

| P5 | 25 | 20 |

| P2 | 33 | 45 |

| P3 | 57 | 78 |

| P1 | 66 | 135 |

+---------+------------+------------+

1 1 1 1 1 2

2 4 6 8 0 2 4 6 8 0

0 0 0 0 0 0 0 0 0 0 0

+----+----+----+----+----+----+----+----+----+----+

| P4 | P5 | P2 | P3 | P1 |

Round Robin (RR), with 20ms quantum

| Process | Burst Time | Allowed Time | Start Time | Time Remaining |

+---------+------------+--------------+------------+----------------+

| P1 | 66 | 20 | 0 | 46 |

| P2 | 33 | 20 | 20 | 13 |

| P3 | 57 | 20 | 40 | 37 |

| P4 | 20 | 20 | 60 | 0 |

| P5 | 25 | 20 | 80 | 5 |

| P1 | 46 | 20 | 100 | 26 |

| P2 | 13 | 13 | 120 | 0 |

| P3 | 37 | 20 | 133 | 17 |

| P5 | 5 | 5 | 153 | 0 |

| P1 | 26 | 20 | 158 | 6 |

| P3 | 17 | 17 | 178 | |

| P1 | 6 | 6 | 195 | 0 |

+---------+------------+--------------+------------+----------------+

1 1 1 1 1 2

2 4 6 8 0 2 4 6 8 0

0 0 0 0 0 0 0 0 0 0 0

+----+----+----+----+----+----+----+----+----+----+

| P1 | P2 | P3 | P4 | P5 | P1 |P2 |P3 ||P1 |P2 ||

P5 P1

b) What is the waiting time of each process for each of the scheduling algorithms in part a?

(See the tables, above.)

c) Which of the schedules in part a results in the minimal average waiting time (over all processes)?

FCFS = Avg(0, 66, 99, 156, 176) = 99.4ms

SJF = Avg(0, 20, 45, 78, 135) = 55.6ms

RR = Avg(0, 20, 40, 60, 80) = 40.0ms