Abraham Silberschatz, Greg Gagne, Peter B. Galvin - Operating System Concepts-Wiley (2018), PART TWO PROCESS MANAGEMENT, Chapter 5 CPU Scheduling, Pg. 202.

Question

 Please read below passage from 10th edition Operating System Concepts, pg. 202:

5.1.3 Preemptive and Nonpreemptive Scheduling

CPU-scheduling decisions may take place under the following four circumstances:

1. When a process switches from the running state to the waiting state (for example, as the result of an I/O request or an invocation of wait() for the termination of a child process)
2. When a process switches from the running state to the ready state (for example, when an interrupt occurs)
3. When a process switches from the waiting state to the ready state (for example, at completion of I/O)
4. When a process terminates

For situations 1 and 4, there is no choice in terms of scheduling. A new process (if one exists in the ready queue) must be selected for execution. There is a choice, however, for situations 2 and 3. When scheduling takes place only under circumstances 1 and 4, we say that the scheduling scheme is nonpreemptive or cooperative. 

My doubt for situation 3 is, Process will inevitably move to ready state once I/O operation is complete as last burst is always CPU burst, so how do we have choice for CPU-scheduling decisions to not take place for situation 3?

Comments

Because, after completion of IO , if any process will have higher priority than the IO completed process then the process having higher priority will have to schedule first by short term scheduler, hence after IO completion process must be put into ready queue to schedule.

Answer 1

It is not necessary that same process will go to ready state after I/O operation.

Process with higher priority could switch the lower priority process.