cpu-scheduling presentation for IT/CS

1. CPU
SCHEDULING
REPOR T
BY:ELFIE LAMPINAQUE
DEXTERJAMES DEGAYO

2. BASIC CONCEPTS
1
SCHEDULING CRITERIA
2
SCHEDULING ALGORITHMS (FCFS,
SJF, PRIORITY, ROUND ROBIN, ETC.)
3
MULTILEVEL QUEUE
SCHEDULING
4
TABLE OF CONTENT

3. BASIC CONCEPTS
CPU SCHEDULING IS A FUNDAMENTAL CONCEPT IN
OPERATING SYSTEMS THAT DETERMINES WHICH
PROCESS GETS TO USE THE CPU WHEN MULTIPLE
PROCESSES ARE READY TO EXECUTE. THE
STRUCTURE OF CPU SCHEDULING INCLUDES
VARIOUS COMPONENTS AND SCHEDULING
ALGORITHMS.

4. CPU SCHEDULING
COMPONENTS
1.PROCESS STATES:
>New Ready Running Waiting Terminated
→ → → →
>The CPU scheduler selects processes from the Ready Queue for
execution.
2.Process Control Block (PCB):
>Stores process attributes like Process ID (PID), CPU registers,
priority, program counter, etc.

5. 3.QUEUES USED IN SCHEDULING:
>Job Queue: Contains all processes in the system.
>Ready Queue: Contains processes ready to execute.
>Waiting Queue: Contains processes waiting for I/O operations.
4.CPU Scheduler
>Selects a process from the Ready Queue and assigns it to the CPU.
5.Dispatcher:
Loads the selected process from the ready queue into the CPU for execution.
Performs context switching and switching to user mode.
CPU SCHEDULING
COMPONENTS

6. TYPES OF CPU SCHEDULING
ALGORITHMS
CPU SCHEDULING CAN BE DIVIDED INTO PREEMPTIVE AND NON-
PREEMPTIVE APPROACHES.
A. Non-Preemptive Scheduling Algorithms
1.First Come First Serve (FCFS):
>Processes are scheduled in the order they arrive.
>Simple but can cause convoy effect (slow process delays faster
ones).
2.Shortest Job Next (SJN) / Shortest Job First (SJF ):
>Process with the smallest burst time is selected first.
>Optimal in minimizing waiting time but suffers from starvation

7. TYPES OF CPU SCHEDULING
ALGORITHMS
3.PRIORITY SCHEDULING:
Process with the highest priority is scheduled first.
Starvation can occur if low-priority processes never get CPU time.
B. PREEMPTIVE SCHEDULING ALGORITHMS
1.ROUND ROBIN (RR):
• EACH PROCESS GETS A FIXED TIME QUANTUM BEFORE SWITCHING
TO THE NEXT.
• REDUCES STARVATION BUT INCREASES CONTEXT SWITCHING
OVERHEAD.
2.SHORTEST REMAINING TIME FIRST (SRTF ):

8. TYPES OF CPU SCHEDULING
ALGORITHMS
3.PRIORITY SCHEDULING (PREEMPTIVE):
• The CPU is assigned to the highest-priority process,
preempting lower-priority ones.
3.Multilevel Queue Scheduling:
• Divides processes into multiple queues based on
priority (e.g., system, interactive, batch).
4.Multilevel Feedback Queue:
• Processes move between queues based on their
behavior (e.g., interactive processes get higher
priority).

9. TO EVALUATE CPU SCHEDULING ALGORITHMS, WE
USE:
• Turnaround Time (TAT) = Completion Time - Arrival
Time
• Waiting Time (WT) = Turnaround Time - Burst Time
• Response Time (RT) = First Response Time -
Arrival Time
• Throughput = Number of processes executed per
unit time
• CPU Utilization = Percentage of CPU time utilized
PERFORMANCE METRICS

10. CPU SCHEDULING IN
MULTIPROCESSOR SYSTEMS
• SYMMETRIC MULTIPROCESSING (SMP): EACH
PROCESSOR HAS ITS OWN SCHEDULER.
• Asymmetric Multiprocessing (AMP): One processor
handles scheduling for all others.

11. 2. SCHEDULING
CRITERIA

12. SCHEDULING CRITERIA
THE SCHEDULING CRITERIA STRUCTURE REFERS
TO THE FACTORS USED TO EVALUATE AND
PRIORITIZE PROCESSES IN A SCHEDULING
ALGORITHM. THESE CRITERIA DETERMINE THE
EFFICIENCY AND EFFECTIVENESS OF A
SCHEDULING SYSTEM, ESPECIALLY IN
OPERATING SYSTEMS AND TASK SCHEDULING.

13. COMMON SCHEDULING CRITERIA:
>CPU UTILIZATION – THE PERCENTAGE OF TIME
THE CPU IS ACTIVELY EXECUTING PROCESSES. A
GOOD SCHEDULER MAXIMIZES CPU USAGE.
>THROUGHPUT – THE NUMBER OF PROCESSES
COMPLETED PER UNIT OF TIME. HIGHER
THROUGHPUT MEANS BETTER PERFORMANCE.

14. COMMON SCHEDULING CRITERIA:
TURNAROUND TIME – THE TOTAL TIME TAKEN
FROM PROCESS SUBMISSION TO COMPLETION,
INCLUDING EXECUTION AND WAITING TIME. THE
GOAL IS TO MINIMIZE TURNAROUND TIME.
WAITING TIME – THE TOTAL TIME A PROCESS
SPENDS WAITING IN THE READY QUEUE BEFORE
EXECUTION. LOWER WAITING TIME IMPROVES
SYSTEM RESPONSIVENESS.

15. RESPONSE TIME – THE TIME
BETWEEN SUBMITTING A REQUEST
AND THE FIRST RESPONSE (NOT
COMPLETION). CRITICAL FOR
INTERACTIVE SYSTEMS.
FAIRNESS – ENSURING ALL
PROCESSES GET A FAIR SHARE OF

16. DEADLINE ADHERENCE – IMPORTANT
FOR REAL-TIME SYSTEMS WHERE
TASKS MUST MEET STRICT DEADLINES.
PRIORITY HANDLING – IF A PRIORITY-
BASED SCHEDULING ALGORITHM IS
USED, IT ENSURES HIGH-PRIORITY
TASKS EXECUTE SOONER.

17. SCHEDULING ALGORITHMS
(FCFS, SJF, PRIORITY,
ROUND ROBIN

18. 1. FIRST COME FIRST SERVE (FCFS)
DESCRIPTION: THE PROCESS THAT
ARRIVES FIRST IS EXECUTED FIRST.
TYPE: NON-PREEMPTIVE
ADVANTAGES: SIMPLE AND EASY TO
IMPLEMENT.
DISADVANTAGES: CAUSES CONVOY
EFFECT (LONG PROCESSES DELAY

19. 3. PRIORITY SCHEDULING
DESCRIPTION: PROCESSES ARE EXECUTED
BASED ON PRIORITY (LOWER VALUE =
HIGHER PRIORITY).
TYPE: CAN BE PREEMPTIVE OR NON-
PREEMPTIVE.
ADVANTAGES: IMPORTANT TASKS GET
EXECUTED FIRST.
DISADVANTAGES: MAY CAUSE STARVATION

20. 2.SHORTEST JOB FIRST (SJF )
DESCRIPTION: THE PROCESS WITH THE
SHORTEST BURST TIME IS EXECUTED
FIRST.
TYPE: CAN BE PREEMPTIVE (SHORTEST
REMAINING TIME FIRST - SRTF ) OR NON-
PREEMPTIVE.
ADVANTAGES: MINIMIZES AVERAGE
WAITING TIME.

21. SCHEDULING ALGORITHMS ARE USED BY
OPERATING SYSTEMS TO DETERMINE THE
ORDER IN WHICH PROCESSES EXECUTE IN A
CPU. THE GOAL IS TO OPTIMIZE CPU
UTILIZATION, RESPONSE TIME, AND
THROUGHPUT WHILE MINIMIZING WAITING
SCHEDULING ALGORITHMS
(FCFS, SJF, PRIORITY, ROUND
ROBIN

22. 1. FIRST-COME, FIRST-SERVED (FCFS)
SCHEDULING
✅ TYPE: NON-PREEMPTIVE
✅ SELECTION CRITERIA: PROCESSES ARE
EXECUTED IN THE ORDER THEY ARRIVE.
✅ STRUCTURE:
MAINTAIN A QUEUE (FIFO – FIRST IN,
FIRST OUT).
THE FIRST PROCESS IN THE QUEUE GETS

23. ✅ ADVANTAGES:
✔ SIMPLE TO IMPLEMENT
✔ NO STARVATION
✅ DISADVANTAGES:
✖ HIGH WAITING TIME IF A LONG
PROCESS COMES FIRST (CONVOY
EFFECT)
✖ NOT SUITABLE FOR TIME-SHARING

24. 2. SHORTEST JOB FIRST (SJF ) SCHEDULING
✅ TYPE: CAN BE PREEMPTIVE (SHORTEST
REMAINING TIME FIRST - SRTF ) OR NON-
PREEMPTIVE
✅ SELECTION CRITERIA: THE PROCESS WITH THE
SHORTEST BURST TIME EXECUTES FIRST.
✅ STRUCTURE:
SORT ALL PROCESSES BASED ON BURST TIME.
THE PROCESS WITH THE LOWEST BURST TIME GETS
CPU FIRST.
IN PREEMPTIVE SJF (SRTF ), IF A NEW PROCESS

25. ✅ ADVANTAGES:
✔ MINIMIZES WAITING TIME AND
TURNAROUND TIME
✔ EFFICIENT FOR BATCH PROCESSING
✅ DISADVANTAGES:
✖ REQUIRES KNOWING BURST TIME IN
ADVANCE
✖ CAN LEAD TO STARVATION OF

26. 3. PRIORITY SCHEDULING
✅ TYPE: CAN BE PREEMPTIVE OR NON-PREEMPTIVE
✅ SELECTION CRITERIA: EACH PROCESS HAS A
PRIORITY NUMBER; THE LOWER THE NUMBER,
THE HIGHER THE PRIORITY.
✅ STRUCTURE:
SORT PROCESSES BASED ON PRIORITY.
THE HIGHEST-PRIORITY PROCESS EXECUTES FIRST.
IN PREEMPTIVE PRIORITY SCHEDULING, A NEW
HIGH-PRIORITY PROCESS INTERRUPTS THE
RUNNING PROCESS.

27. ✅ ADVANTAGES:
✔ USEFUL FOR REAL-TIME SYSTEMS
✔ ENSURES URGENT TASKS GET CPU
FIRST
✅ DISADVANTAGES:
✖ LOW-PRIORITY PROCESSES MAY STARVE
(STARVATION PROBLEM)
✖ REQUIRES A MECHANISM (AGING) TO
PREVENT STARVATION

28. 4. ROUND ROBIN (RR) SCHEDULING
✅ TYPE: PREEMPTIVE
✅ SELECTION CRITERIA: EACH PROCESS GETS A
FIXED TIME SLICE (QUANTUM).
✅ STRUCTURE:
EACH PROCESS IS GIVEN A TIME SLICE (E.G.,
4MS).
AFTER ITS TIME SLICE, THE PROCESS IS MOVED
TO THE BACK OF THE QUEUE IF IT HASN’ T
FINISHED.

29. ✅ ADVANTAGES:
✔ FAIR FOR ALL PROCESSES
✔ GOOD FOR TIME-SHARING
SYSTEMS
✅ DISADVANTAGES:
✖ CONTEXT SWITCHING OVERHEAD
✖ HIGHER TURNAROUND TIME FOR
LONG PROCESSES IF THE QUANTUM

31. MULTILEVEL QUEUE
SCHEDULING

32. WHAT IS MULTILEVEL QUEUE SCHEDULING
MULTILEVEL QUEUE (MLQ) SCHEDULING IS A
CPU SCHEDULING ALGORITHM THAT DIVIDES
PROCESSES INTO MULTIPLE QUEUES BASED
ON THEIR CHARACTERISTICS (E.G., PRIORITY,
PROCESS TYPE, OR MEMORY REQUIREMENTS).
EACH QUEUE HAS ITS OWN SCHEDULING
ALGORITHM, AND PROCESSES DO NOT MOVE

33. STRUCTURE OF MULTILEVEL QUEUE SCHEDULING
✅ KEY FEATURES:
DIVIDES THE READY QUEUE INTO MULTIPLE QUEUES.
EACH QUEUE HAS ITS OWN SCHEDULING ALGORITHM
(E.G., FCFS, SJF, RR).
A SCHEDULING POLICY DETERMINES HOW CPU TIME
IS ALLOCATED BETWEEN QUEUES.
✅ TYPES OF SCHEDULING BETWEEN QUEUES:
FIXED PRIORITY SCHEDULING: HIGHER-PRIORITY
QUEUES ALWAYS EXECUTE FIRST.
TIME-SLICE SCHEDULING: CPU TIME IS DIVIDED

34. TYPICAL QUEUES IN MLQ
SCHEDULING

35. ADVANTAGES & DISADVANTAGES
✅ ADVANTAGES:
✔ PRIORITIZES CRITICAL TASKS (SYSTEM AND
INTERACTIVE PROCESSES).
✔ EFFICIENT USE OF CPU BY ALLOCATING DIFFERENT
SCHEDULING ALGORITHMS.
✔ GOOD FOR SYSTEMS WITH DIVERSE WORKLOAD TYPES.
❌ DISADVANTAGES:
✖ STARVATION CAN OCCUR IF LOWER-PRIORITY QUEUES
GET LIT TLE CPU TIME.
✖ PROCESSES ARE STUCK IN THEIR QUEUES (NO
MOVEMENT BETWEEN QUEUES).
✖ COMPLEX TO IMPLEMENT.

36. WHEN TO USE MULTILEVEL
QUEUE SCHEDULING?
• OPERATING SYSTEMS: SEPARATING SYSTEM
PROCESSES, USER APPLICATIONS, AND
BACKGROUND TASKS.
• CLOUD COMPUTING: ASSIGNING PRIORITY TO
DIFFERENT WORKLOADS (REAL-TIME
PROCESSING VS. BATCH JOBS).

37. THANK
YOU