Priority scheduling in Operating Systems - Step-by-Step Execution

Concept Flow - Priority scheduling

Start: Processes arrive

↓

Assign priority to each process

↓

Check ready queue

↓

Select process with highest priority

↓

Execute selected process

↓

Process completes or is preempted

↓

Update ready queue

↩Back to Check ready queue

Priority scheduling picks the process with the highest priority from the ready queue to run next, repeating this until all processes finish.

Execution Sample

Operating Systems

Processes = [P1(priority=3), P2(priority=1), P3(priority=4)]
ReadyQueue = [P1, P2, P3]
while ReadyQueue not empty:
  Pick process with highest priority
  Run process
  Remove process from ReadyQueue

This example shows selecting and running processes based on their priority until none remain.

Analysis Table

Step	Ready Queue	Selected Process	Reason	Action	Remaining Queue
1	[P1(3), P2(1), P3(4)]	P2(1)	P2 has highest priority (lowest number)	Run P2	[P1(3), P3(4)]
2	[P1(3), P3(4)]	P1(3)	P1 has next highest priority	Run P1	[P3(4)]
3	[P3(4)]	P3(4)	Only process left	Run P3	[]
4	[]	-	No processes left	Stop scheduling	[]

💡 Ready queue is empty, all processes have been executed.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
ReadyQueue	[P1(3), P2(1), P3(4)]	[P1(3), P3(4)]	[P3(4)]	[]	[]
SelectedProcess	-	P2(1)	P1(3)	P3(4)	-

Key Insights - 3 Insights

Why is P2 selected first even though it appears second in the queue?

Because priority scheduling selects the process with the highest priority (lowest priority number), not the order in the queue. See execution_table step 1.

What happens when the ready queue becomes empty?

Scheduling stops because there are no more processes to run, as shown in execution_table step 4.

Does priority scheduling always run processes in arrival order?

No, it runs processes based on priority, not arrival order. The process with the highest priority runs first regardless of arrival time.

Visual Quiz - 3 Questions

Look at the execution table. Which process is selected at step 2?

A P3

B P2

C P1

D No process

💡 Hint

Check the 'Selected Process' column at step 2 in the execution_table.

At which step does the ready queue become empty?

A Step 1

B Step 4

C Step 3

D Never

💡 Hint

Look at the 'Ready Queue' column and find when it shows [].

If P3 had priority 0 (highest), which process would run first?

A P3

B P2

C P1

D None

💡 Hint

Priority scheduling picks the process with the lowest priority number first, see variable_tracker for priority values.

Concept Snapshot

Priority Scheduling:
- Assign priority number to each process (lower number = higher priority).
- Always pick the process with highest priority to run next.
- Continue until all processes finish.
- Does not necessarily follow arrival order.
- Can be preemptive or non-preemptive.

Full Transcript

Priority scheduling is a method where the operating system selects the process with the highest priority to run next. Each process is assigned a priority number; the lower the number, the higher the priority. The scheduler looks at the ready queue, picks the process with the highest priority, runs it, then removes it from the queue. This repeats until no processes remain. The execution table shows step-by-step which process runs and how the queue changes. Key points include that priority, not arrival order, determines execution, and scheduling stops when the queue is empty.

Practice

1. What does priority scheduling in operating systems primarily use to decide which process runs first?

easy

A. The time each process has already run

B. The importance level assigned to each process

C. The size of the process in memory

D. The order in which processes arrive

Solution

1. Step 1: Understand priority scheduling basics

   Priority scheduling chooses processes based on their assigned importance or priority level.

2. Step 2: Compare with other scheduling criteria

   Unlike first-come-first-served or shortest job first, priority scheduling uses priority, not arrival time or size.

3. Final Answer:

   The importance level assigned to each process -> Option B

4. Quick Check:

   Priority scheduling = importance level [OK]

Hint: Remember: priority means importance, not arrival time [OK]

Common Mistakes:

• Confusing priority with arrival order
• Thinking process size affects scheduling
• Assuming time already run decides priority

2. Which of the following is the correct way to describe a preemptive priority scheduling system?

easy

A. A system that ignores priority and runs processes randomly

B. A system where processes run to completion without interruption

C. A system that schedules processes based on their arrival time only

D. A system where a running process can be interrupted if a higher priority process arrives

Solution

1. Step 1: Define preemptive scheduling

   Preemptive scheduling allows interruption of a running process if a more important one arrives.

2. Step 2: Match with priority scheduling

   In priority scheduling, preemptive means higher priority processes can interrupt lower priority ones.

3. Final Answer:

   A system where a running process can be interrupted if a higher priority process arrives -> Option D

4. Quick Check:

   Preemptive priority = interrupt for higher priority [OK]

Hint: Preemptive means interrupt if higher priority comes [OK]

Common Mistakes:

• Confusing preemptive with non-preemptive
• Thinking processes always run to completion
• Ignoring priority in scheduling decisions

3. Consider three processes with priorities: P1=2, P2=1, P3=3 (1 is highest priority). If all arrive at the same time, which order will they be scheduled in a non-preemptive priority scheduling?

medium

A. P3, P1, P2

B. P1, P2, P3

C. P2, P1, P3

D. P1, P3, P2

Solution

1. Step 1: Identify priority order

   Priority 1 is highest, so P2 (priority 1) runs first, then P1 (2), then P3 (3).

2. Step 2: Apply non-preemptive scheduling

   Since all arrive together, processes run fully in priority order without interruption.

3. Final Answer:

   P2, P1, P3 -> Option C

4. Quick Check:

   Lower number = higher priority, run in that order [OK]

Hint: Lower priority number runs first in non-preemptive [OK]

Common Mistakes:

• Mixing priority numbers with arrival order
• Assuming preemption changes order here
• Confusing priority 1 as lowest priority

4. A priority scheduling system is implemented but processes with lower priority sometimes run before higher priority ones. What is the most likely cause?

medium

A. The system is using non-preemptive scheduling and a low priority process started first

B. The priority numbers are assigned incorrectly with higher numbers meaning higher priority

C. The system is ignoring arrival times completely

D. The CPU is idle and no processes are running

Solution

1. Step 1: Understand non-preemptive behavior

   In non-preemptive priority scheduling, once a process starts, it runs to completion even if a higher priority process arrives later.

2. Step 2: Explain why lower priority runs first

   If a low priority process starts first, it will finish before the higher priority process can run.

3. Final Answer:

   The system is using non-preemptive scheduling and a low priority process started first -> Option A

4. Quick Check:

   Non-preemptive lets running process finish first [OK]

Hint: Non-preemptive means no interruption once started [OK]

Common Mistakes:

• Assuming priority numbers are reversed
• Ignoring scheduling type (preemptive vs non-preemptive)
• Thinking arrival time is always ignored

5. You have four processes arriving at different times with priorities: P1(2, arrives at 0s), P2(1, arrives at 1s), P3(3, arrives at 2s), P4(1, arrives at 3s). Using preemptive priority scheduling, which process runs at time 3 seconds?

hard

A. P2

B. P4

C. P1

D. P3

Solution

1. Step 1: Track process arrivals and priorities

   At 0s: P1(2) starts. At 1s: P2(1) arrives and preempts P1 since higher priority (lower number). At 2s: P3(3) arrives, lower priority than P2(1), so P2 continues. At 3s: P4(1) arrives, same priority as running P2.

2. Step 2: Determine which process runs at 3s

   In preemptive priority scheduling, preemption occurs only if a strictly higher priority process arrives. Since P4 has the same priority (1) as P2, P2 is not preempted and continues running at time 3s.

3. Final Answer:

   P2 -> Option A

4. Quick Check:

   Preempt only for higher priority; same priority continues [OK]

Hint: Preempt only if strictly higher priority [OK]

Common Mistakes:

• Thinking same priority causes preemption
• Assuming new arrivals always preempt
• Confusing tie-breaker rules (usually FCFS for same priority)