Question

Suppose in a multiprogramming environment, the following C program segment is executed. A process goes into the I/O queue whenever an I/O related operation is performed. Assume that there will always be a context switch whenever a process requests an I/O, and also whenever the process returns from an I/O. The number of times the process will enter the ready queue during its lifetime (not counting the time the process enters the ready queue when it is run initially) is _________ (Answer in integer).

Hint:

Each I/O operation (like `scanf()` and `printf()`) causes a context switch, where the process moves between the I/O queue and the ready queue. Count the number of iterations and operations to determine the number of context switches.

Answer 1

Correct Answer: 21

Problem Analysis:

To determine the number of times a process enters the Ready Queue, we must track every transition from the Waiting (I/O) State back to the Ready State. According to the problem constraints: 

• An I/O operation moves the process to the I/O queue (Waiting state).
• Returning from an I/O operation triggers a context switch, moving the process back to the Ready Queue.
• Initial entry into the ready queue (when the process is first created) is not counted.

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Step 1: Identify I/O Operations in the Code

Standard C library functions like scanf() and printf() are I/O related operations. Let's count their occurrences in the program execution:

• scanf("%d", &x);: This occurs once before the loop starts.
• printf("%d\n", x);: This resides inside a for loop that iterates from i = 0 to i < 20. Thus, it executes 20 times.

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Step 2: Map I/O Completion to Ready Queue Entries

Every time an I/O operation completes, the process is moved from the I/O queue to the Ready Queue. We sum the completion of all I/O calls identified in Step 1:

• Entry from 1st I/O (scanf) = 1
• Entries from loop I/O (printf) = 20

Total Ready Queue entries = $1 + 20 = 21$.

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Final Answer:

The number of times the process will enter the ready queue during its lifetime (not counting the initial run) is:21