Question

Three CPU-bound tasks, with execution times of 15, 12 and 5 time units respectively arrive at times 0, t and 8, respectively. If the operating system implements a shortest remaining time first scheduling algorithm, what should be the value of t to have 4 context switches?

Ignore the context switches at time 0 and at the end.
1. 0 < t < 3
2. t = 0
3. t < = 3
4. 3 < t < 8

Answer 1

Correct Answer - Option 1 : 0 < t < 3

Process scheduling Algorithm: shortest remaining time first scheduling

Process Table:

Process	Arrival Time	Burst Time
P0	0	15
P1	t	12
P2	8	5

 

Gantt Chart:

At t = 0:

P1

P1

P2

P0

0           8            12          17          32

                           1^st              2^nd 

2 context switches occur. Hence Option 2  and 3 is eliminated.

At t = 1:

P0

P1

P2

P1

P0

0           1            8           13           18         32

              1^st        2^nd         3^rd          4^th   

4 context switches occur.

Since P₂ and P₁ has the same burst time we can choose any process at t = 8

At t = 2:

P0

P1

P2

P1

P0

0           2            8           13           19          32

             1^st         2^nd         3^rd          4^th  

4 context switches occur.

At = 5:

P0

P0

P2

P0

P1

0           5            8           13           20          32

                           1^st         2^nd          3^rd  

Only 3 context switches occur. Hence Option 4 is also eliminated.

Therefore, option 1 is the correct answer