Round Robin scheduling in Operating Systems - Step-by-Step Execution

Concept Flow - Round Robin scheduling

Start with ready queue

↓

Pick first process

↓

Run process for time quantum

↓

Process finished?

Yes→Remove from queue

No↓

Move process to end of queue

↓

More processes in queue?

Yes→Pick next process

No↓

End

Round Robin scheduling cycles through processes, giving each a fixed time slice, then moves unfinished processes to the queue's end until all finish.

Execution Sample

Operating Systems

Processes = [P1(5), P2(3), P3(7)]
Time Quantum = 2
While processes remain:
  Run current process for 2 units
  If done, remove
  Else, move to end

This simulates running three processes with different burst times using a 2-unit time slice each cycle.

Analysis Table

Step	Current Process	Time Run	Remaining Time Before	Remaining Time After	Queue State	Action
1	P1	2	5	3	[P1, P2, P3]	Run P1 for 2, move to end
2	P2	2	3	1	[P2, P3, P1]	Run P2 for 2, move to end
3	P3	2	7	5	[P3, P1, P2]	Run P3 for 2, move to end
4	P1	2	3	1	[P1, P2, P3]	Run P1 for 2, move to end
5	P2	1	1	0	[P2, P3, P1]	Run P2 for 1, finish and remove
6	P3	2	5	3	[P3, P1]	Run P3 for 2, move to end
7	P1	1	1	0	[P1, P3]	Run P1 for 1, finish and remove
8	P3	2	3	1	[P3]	Run P3 for 2, move to end
9	P3	1	1	0	[P3]	Run P3 for 1, finish and remove
10	-	-	-	-	[]	All processes finished, stop

💡 All processes have zero remaining time, queue is empty, scheduling ends.

State Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	Final
P1 Remaining Time	5	3	3	1	1	1	1	0	0	0	0
P2 Remaining Time	3	3	1	1	1	0	0	0	0	0	0
P3 Remaining Time	7	7	7	5	5	5	3	3	1	0	0
Queue State	[P1, P2, P3]	[P2, P3, P1]	[P3, P1, P2]	[P1, P2, P3]	[P2, P3, P1]	[P3, P1]	[P1, P3]	[P3]	[P3]	[]	[]

Key Insights - 3 Insights

Why does a process move to the end of the queue after its time quantum?

If the process is not finished after its time quantum (see steps 1, 2, 3), it must wait its turn again, so it moves to the queue's end to allow others to run.

What happens when a process finishes before using the full time quantum?

At step 5, P2 only needs 1 unit to finish, so it runs less than the quantum and is removed immediately from the queue.

How do we know when the scheduling ends?

When the queue is empty (step 10), all processes have finished, so the scheduler stops running.

Visual Quiz - 3 Questions

Look at the execution table, what is P1's remaining time after step 4?

A 3

B 0

C 1

D 5

💡 Hint

Check the 'Remaining Time After' column for P1 at step 4 in the execution_table.

At which step does P2 finish execution?

A Step 5

B Step 4

C Step 7

D Step 9

💡 Hint

Look for when P2's remaining time becomes 0 in the execution_table.

If the time quantum was increased to 3, how would the queue state after step 1 change?

A [P2, P3, P1]

B [P3, P1, P2]

C [P1, P2, P3]

D [P2, P1, P3]

💡 Hint

With a 3-unit quantum, P1 would run for 3 units, reducing its remaining time from 5 to 2, then move to the end; check how this affects queue order.

Concept Snapshot

Round Robin Scheduling:
- Each process runs for a fixed time slice (quantum).
- If unfinished, process moves to queue end.
- Continues cycling until all finish.
- Fair and simple for time-sharing systems.
- Prevents any process from starving.
- Time quantum size affects responsiveness and overhead.

Full Transcript

Round Robin scheduling works by cycling through all processes in the ready queue, giving each a fixed time slice to run. If a process finishes within its time slice, it is removed from the queue. If not, it is moved to the end of the queue to wait for its next turn. This continues until all processes have completed. The execution table shows step-by-step how processes run, their remaining times, and how the queue changes. Key moments include understanding why processes move to the queue's end, what happens when a process finishes early, and when the scheduling ends. The visual quiz tests understanding of remaining times, finishing steps, and effects of changing the time quantum.