Question

Assume there are three processes A, B and C. All processes are submitted for execution at the same time. The process A executes for 2 milliseconds and then does I/O for 3 milliseconds and then executes for 3 milliseconds more. The process B executes for 4 milliseconds and then does I/O for 7 milliseconds. The process C executes for 2 millisecond and then does I/O for 4 milliseconds. Assume no contention for the resources and the order of execution (priority) is A, B and C.

Calculate the processor utilization with multiprogramming.

Answer 1

Step 1: Given that, there are three processes A, B, C.

Process A executes for 2ms and then does I/O for 3ms then executes for 3ms.

Process B executes for 4ms and then does I/O for 7ms.

Process C executes for 2ms and then does I/O for 4ms.

Priority for execution is A,B,C.

Step 2: In a multiprogramming environment, when a process is in I/O, the OS will schedule another process to CPU for execution. Lets see the time line of the three processes in a multiprogramming environment:

Process/timeline	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
Process A
Process B
Process C

The red color cells indicate process execution and the yellow cells indicate I/O.

Step 3: CPU Utilization = Time spent by CPU for execution / (Time spent by CPU for execution + CPU Idle time)

Here CPU idle time includes the time in which processes are only in I/O.

Time spent by CPU for execution = (2+3)+4+2 = 11ms

CPU Idle time = 9ms

Step 4: CPU Utilization = 11 / (11+9) = 0.55

Hence processor utilization with multiprogramming is 55%.