Operating System

Describe round-robin (RR) CPU scheduling ?

Round-robin (RR) scheduling is a preemptive scheduling algorithm that allocates each task an equal share of CPU time. Tasks are placed in a circular queue and when a task’s allocated CPU time expires, it is moved to the end of the queue. The next task is then taken from the front of the queue. Round-robin scheduling can be used in embedded systems. In this context, the operating system is driven by a regular interrupt, called a “clock tick”. Tasks are selected in a fixed sequence for execution.

RR scheduling is commonly used in operating systems to schedule processes. It is a fair algorithm that ensures that all processes get an equal share of CPU time. However, it can be inefficient if there are a large number of processes, as each process will only get a small amount of CPU time before being preempted.

Here is an example of RR scheduling:

  • There are four processes, P1, P2, P3, and P4.
  • Each process is assigned a time quantum of 10 milliseconds.
  • P1 runs for 10 milliseconds, then P2 runs for 10 milliseconds, then P3 runs for 10 milliseconds, and then P4 runs for 10 milliseconds.
  • After P4 has run, P1 runs again, and so on.
  • This process continues until all processes have finished running.

RR scheduling is a simple and effective algorithm that can be used to schedule processes in a fair and efficient manner.

Here is an example of round robin scheduling:

Suppose there are three processes, P1, P2, and P3, with burst times of 5, 9, and 15 milliseconds, respectively. If the time quantum is 2 milliseconds, then the following will happen:

  • At time 0, P1 begins execution.
  • At time 2, P1 is interrupted and P2 begins execution.
  • At time 4, P2 is interrupted and P3 begins execution.
  • At time 6, P3 is interrupted and P1 begins execution.
  • At time 8, P1 is interrupted and P2 begins execution.
  • At time 10, P2 is interrupted and P3 begins execution.
  • At time 12, P3 completes execution.
  • At time 14, P1 completes execution.
  • At time 16, P2 completes execution.

The average completion time for this example is (12 + 14 + 16) / 3 = 14 milliseconds.

Round robin scheduling is a fair algorithm that ensures that all processes get an equal share of CPU time. However, it can lead to longer waiting times for processes with longer burst times.

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