Multilevel queue scheduling in Operating Systems - Step-by-Step Execution

Concept Flow - Multilevel queue scheduling

New process arrives

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Assign to a queue based on priority/type

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Select queue to schedule

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Schedule process from selected queue

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Process runs

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Process finishes or yields

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Repeat scheduling from queues

Processes are divided into separate queues by type or priority. The scheduler picks a queue and runs processes from it, repeating this cycle.

Execution Sample

Operating Systems

Queues:
  Q1: System processes (highest priority)
  Q2: Interactive processes
  Q3: Batch processes (lowest priority)
Schedule order: Q1 -> Q2 -> Q3
Run one process from each queue in turn.

This example shows three queues with different priorities. The scheduler runs one process from the highest priority queue first, then moves to the next queue.

Analysis Table

Step	Queue Selected	Process Chosen	Action	Result
1	Q1	P1 (System)	Run process	P1 runs
2	Q2	P2 (Interactive)	Run process	P2 runs
3	Q3	P3 (Batch)	Run process	P3 runs
4	Q1	P4 (System)	Run process	P4 runs
5	Q2	P5 (Interactive)	Run process	P5 runs
6	Q3	P6 (Batch)	Run process	P6 runs
7	Q1	No process	Skip queue	Move to Q2
8	Q2	No process	Skip queue	Move to Q3
9	Q3	No process	Skip queue	No processes left, scheduling ends

💡 All queues are empty, no processes left to schedule.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
Q1 Processes	[P1, P4]	[P4]	[P4]	[P4]	[]	[]	[]	[]	[]	[]
Q2 Processes	[P2, P5]	[P2, P5]	[P5]	[P5]	[P5]	[]	[]	[]	[]	[]
Q3 Processes	[P3, P6]	[P3, P6]	[P3, P6]	[P6]	[P6]	[P6]	[]	[]	[]	[]

Key Insights - 3 Insights

Why does the scheduler skip a queue when it has no processes?

Because the execution_table rows 7 and 8 show 'No process' and 'Skip queue', the scheduler moves on to the next queue to avoid waiting on empty queues.

How does the scheduler decide which queue to pick next?

The scheduler follows a fixed order of queues by priority, as shown in the concept_flow and execution_table, cycling through queues from highest to lowest priority.

What happens if a process in a lower priority queue is ready but higher priority queues are not empty?

The scheduler cycles through the queues in fixed order, so the lower priority process gets its turn even if higher priority queues have processes ready, as seen in step 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, which process runs and from which queue?

A P3 from Q3

B P2 from Q2

C P1 from Q1

D P4 from Q1

💡 Hint

Check the 'Queue Selected' and 'Process Chosen' columns at step 3 in the execution_table.

At which step does the scheduler find Q1 empty and skip it?

A Step 9

B Step 5

C Step 7

D Step 3

💡 Hint

Look for 'No process' and 'Skip queue' in the execution_table for Q1.

If a new system process arrives after step 9, what will change in the variable_tracker?

A Q3 Processes will have a new process added

B Q1 Processes will have a new process added

C Q2 Processes will have a new process added

D No changes because scheduling ended

💡 Hint

System processes belong to Q1 as shown in the variable_tracker start state.

Concept Snapshot

Multilevel queue scheduling divides processes into separate queues by priority or type.
Scheduler picks queues in fixed order, running processes from each queue in turn.
Higher priority queues run before lower priority ones.
Queues are independent; processes do not move between queues.
If a queue is empty, scheduler skips it and moves on.
Scheduling ends when all queues are empty.

Full Transcript

Multilevel queue scheduling organizes processes into different queues based on their priority or type. When a new process arrives, it is assigned to a specific queue. The scheduler then selects queues in a fixed priority order, running one process from each queue in turn. If a queue has no processes, the scheduler skips it and moves to the next queue. This cycle repeats until all queues are empty and no processes remain to be scheduled. This method ensures that higher priority processes get CPU time before lower priority ones, but processes do not move between queues once assigned.