1. Operating Systems (CS 340 D)
Princess Nora University
Faculty of Computer & Information Systems
Computer science Department
Operating Systems (CS 340 D)
Dr. Abeer Mahmoud

2. (Chapter-6) CPU Scheduling

3. Chapter 5: CPU Scheduling
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Dr. Abeer Mahmoud

4. To describe various CPU-scheduling algorithms
OBJECTIVES:
To introduce CPU scheduling, which is the basis for multiprogrammed operating systems
To describe various CPU-scheduling algorithms
Dr. Abeer Mahmoud

5. Basic Concepts
Dr. Abeer Mahmoud

6. Basic Concepts
Maximum CPU utilization (keep the CPU as busy as possible ) obtained with multiprogramming
CPU–I/O Burst Cycle
Process execution consists of a cycle of CPU execution and I/O wait
Process execution begins with a CPU burst…That is followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on.
Eventually, the final CPU burst ends with a system request to terminate execution
Dr. Abeer Mahmoud

7. Histogram of CPU-burst Times
The durations of CPU bursts vary greatly from process to process.
There is large number of short CPU bursts and a small number of long CPU bursts.
An I/O-bound program >>>> has many short CPU bursts.
A CPU-bound program >>>> has a few long CPU bursts
Dr. Abeer Mahmoud

8. CPU Scheduler CPU Scheduler (/ short-term scheduler):
Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them
The ready queue is not necessarily a first-in, first-out (FIFO) queue. It can be implemented as a FIFO queue, a priority queue, a tree, or an unordered linked list.
Dr. Abeer Mahmoud

9. Preemptive Scheduling
CPU scheduling decisions may take place when a process:
Switches from running to waiting state
Switches from running to ready state
Switches from waiting to ready
Terminates
Scheduling under 1 and 4 is non-preemptive (/cooperative).
scheduling under 2 and 3 is preemptive
Dr. Abeer Mahmoud

10. Preemptive Scheduling (cont..)
Under non-preemptive scheduling,
once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state
Dr. Abeer Mahmoud

11. Dispatcher
Dispatcher : is the module gives control of the CPU to the process selected by the short-term scheduler.
The dispatcher should be as fast as possible, since it is invoked during every process switch.
Dispatch latency – time it takes for the dispatcher to stop one process and start another running
Dr. Abeer Mahmoud

12. Scheduling Criteria
Dr. Abeer Mahmoud

13. Scheduling Criteria
Throughput – # of processes that complete their execution per time unit (e.g. 10 processes /sec)
Turnaround time – amount of time to execute a particular process (the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O.)
Dr. Abeer Mahmoud

14. Scheduling Criteria (cont)
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is produced (for time-sharing environment)
Dr. Abeer Mahmoud

15. Scheduling Algorithm Optimization Criteria
Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time
Dr. Abeer Mahmoud

16. Scheduling Algorithms
Dr. Abeer Mahmoud

17. Scheduling Algorithms
First-Come, First-Served Scheduling
Shortest-Job-First Scheduling
Priority Scheduling
Round-Robin Scheduling
Multilevel Queue scheduling
Multilevel Feedback Queue
Dr. Abeer Mahmoud

18. (1) First-Come, First-Served (FCFS)
The simplest CPU-scheduling algorithm
The process that requests the CPU first is allocated first.
Can be implemented using FIFO queue:
When a process enters the ready queue, its PCB is linked onto the tail of the queue. When the CPU is free, it is allocated to the process at the head of the queue. The running process is then removed from the queue.
FCFS algorithm is non-preemptive
Dr. Abeer Mahmoud

19. (1) First-Come, First-Served (FCFS) -cont..
Gantt chart: is a bar chart that illustrates a particular schedule, including the start and finish times of each of the processes.
Example(1): Consider the following set of processes that arrive at time 0,with the length of the CPU burst given in milliseconds
Process Burst Time(ms) P P P
Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( )/3 = 17 P1 P2 P3 24 27 30
Dr. Abeer Mahmoud

20. (1) First-Come, First-Served (FCFS) -cont..
Example(2): Consider the same previous set of processes arrive at time 0,with the length of the CPU burst in milliseconds
Process Burst Time(ms)
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P2 , P3 , P1
The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: ( )/3 = 3 >>>>Much better than example (1) P1 P3 P2 6 3 30
Dr. Abeer Mahmoud

21. (1) First-Come, First-Served (FCFS) -cont..
Convoy effect - short processes wait for the one big process to get off the CPU. -This effect results in lower CPU and device utilization than might be possible if the shorter processes were allowed to go first.
FCFS Pros. (++):
Simplest algorithm
FCFS Cons. (--):
The average waiting time is generally not minimal and affected by processes’ order.
Lower CPU and device utilization because of convoy effect
Not suitable for time-shared systems
Dr. Abeer Mahmoud

22. (2) Shortest-Job-First (SJF) Scheduling
Associate with each process the length of its next CPU burst.
Use these lengths to schedule the process with the shortest time
If the next CPU bursts of two processes are the same, FCFS scheduling is used to select the next process
Two schemes:
Non-preemptive
–Preemptive
once CPU given to the process it cannot be preempted until completes its CPU burst
if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)
Dr. Abeer Mahmoud

23. (2) Shortest-Job-First (SJF) Scheduling
Example(3): Consider the following set of processes with the length of the CPU burst given in milliseconds
Process Burst Time(ms)
P1 6
P2 8
P3 7
P4 3
SJF scheduling chart
Average waiting time = ( ) / 4 = 7 P4 P3 P1 3 16 9 P2 24
Dr. Abeer Mahmoud

24. (2) Shortest-Job-First (SJF) Scheduling
Example(4): Consider the following set of processes with the length of the CPU burst given in milliseconds
Process Arrival Time Burst Time P P P P
Non-preemptive SJF
Average waiting time = ( )/4 = 4 P1 P3 P2 7 3 16 P4 8 12
Dr. Abeer Mahmoud

25. (2) Shortest-Job-First (SJF) Scheduling
Example(5): Consider the following set of processes with the length of the CPU burst given in milliseconds
Process Arrival Time Burst Time P P P P
Preemptive SJF
Average waiting time = ( )/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16
Dr. Abeer Mahmoud

26. (2) Shortest-Job-First (SJF) Scheduling
SJF Pros. (++):
SJF is optimal – gives minimum average waiting time for a given set of processes
SJF Cons. (--):
The difficulty is knowing the length of the next CPU request ( some times this time is predicted)
Dr. Abeer Mahmoud

27. (3) Priority Scheduling (cont..)
A priority number (integer) is associated with each process. The CPU is allocated to the process with the highest priority
Equal-priority processes are scheduled in FCFS order.
Text book assumes (smallest integer  highest priority)
Two schemes:
Non-preemptive
–Preemptive
once CPU given to the process it cannot be preempted until completes its CPU burst
if a new process arrives with priorty higher of current executing process, preempt
Dr. Abeer Mahmoud

28. (3) Priority Scheduling (cont..)
Example(6): Consider the following set of processes with the length of the CPU burst given in milliseconds
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5 P
Priority scheduling chart
Average waiting time = ( )/5 = 8.2 ms
Dr. Abeer Mahmoud

29. (3) Priority Scheduling (cont..)
Priority scheduling Pros. (++):
Simple algorithm
Priority scheduling Cons. (--):
Main Problem - Starvation ( /indefinite blocking) low priority processes may never execute
Solution >> Aging >>as time progresses increase the priority of the process
Dr. Abeer Mahmoud

30. (4) Round Robin Scheduling (RR)
(RR) algorithm is designed especially for timesharing systems.
Each process gets a small unit of CPU time (time quantum ).
After this time has elapsed, the process is preempted and added to the end of the ready queue.
Time quantum (/ time slice ) (q)>> usually ms.
The ready queue is treated as a circular queue and implemented as FIFO queue
Dr. Abeer Mahmoud

31. (4) Round Robin Scheduling (RR)
Example(6): Consider the following set of processes with the length of the CPU burst given in milliseconds
time quantum = 4 ms
Process Burst Time
P1 24
P2 3
P3 3
The Gantt chart is:
Average waiting time = ( )/3 = 5.66 ms P1 P2 P3 4 7 10 14 18 22 26 30
Dr. Abeer Mahmoud

32. (4) Round Robin Scheduling (RR)
If there are n processes in the ready queue and the time quantum is q, then :
Each process gets 1/n of the CPU time in chunks of at most (q) time units at once.
No process waits more than (n-1) *q time units.
Performance (depends on the size of the time quantum)
If q is very large  RR is same as FCFS
If q is very small  decrease the performance because of context switch time and increase system overhead
Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process.
Dr. Abeer Mahmoud

33. (4) Round Robin Scheduling (RR)
Turnaround time depends on the size of the time quantum.
The average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum.
Dr. Abeer Mahmoud

34. (4) Round Robin Scheduling (RR)
RR Scheduling Pros. (++):
Suitable to time-shared system (better response time)
RR Scheduling Cons. (--):
The average waiting time under the RR policy is often long
Context switch overhead is higher
Dr. Abeer Mahmoud

35. (5) Multilevel Queue Scheduling
Processes are easily classified into different groups, Such as:
Foreground (interactive) processes (may have higher priority)
Background (batch) processes
Ready queue is partitioned into separate queues
The processes are permanently assigned to one queue,
Each queue has its own scheduling algorithm.
E.g.: foreground processes >> scheduled by RR & background process >> scheduled by FCFS
Scheduling must be done between the queues.
Dr. Abeer Mahmoud

36. (5) Multilevel Queue Scheduling
Example (8): A multilevel queue scheduling algorithm with five queues, listed in order of priority
Each queue has absolute priority over lower-priority queues.
E.g. No process in the batch queue could run unless the queues for system processes, interactive processes, and interactive editing processes were all empty.
If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted.
Dr. Abeer Mahmoud

37. (5) Multilevel Queue Scheduling
Multilevel Queue Scheduling Pros. (++):
Low scheduling overhead
Consider different process prosperities & requirements
Multilevel Queue Scheduling Cons. (--):
Inflexible: a process can’t change it’s queue
Starvation possibility
Dr. Abeer Mahmoud

38. (6) Multilevel Feedback Queue
A process can move between the various queues
The idea is to separate processes according to the characteristics of their CPU bursts.
If a process uses too much CPU time, it will be move to a lower- priority queue.
This scheme leaves I/O-bound and interactive processes in the higher-priority queues.
A process that waits too long in a lower-priority queue may be moved to a higher-priority queue. This form of aging prevents starvation.
Dr. Abeer Mahmoud

39. (6) Multilevel Feedback Queue
Example(9): consider a multilevel
feedback queue scheduler with three queues:
Q0 – RR with time quantum 8 milliseconds (higher priority)
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling
Processes in lower priority queue is selected if the higher queues are empty
A new job enters queue Q0 which is served RR. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
If Q0 is empty, process at Q1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
Q0 ( highest priority) Q1
Q2 (lowest priority)
Dr. Abeer Mahmoud

40. (6) Multilevel Feedback Queue
Multilevel-feedback-queue scheduler defined by the following parameters:
Number of queues
Scheduling algorithms for each queue
Method used to determine when to upgrade a process
Method used to determine when to demote a process
Method used to determine which queue a process will enter when that process needs service
Dr. Abeer Mahmoud

41. (6) Multilevel Feedback Queue
Multilevel Feedback Queue Scheduling Pros. (++):
Very flexible>>>it is the most general CPU-scheduling algorithm.
Can be configured to prevent starvation.
Multilevel Feedback Queue Scheduling Cons. (--):
Most complex algorithm
Dr. Abeer Mahmoud

42. Multiple-Processor Scheduling
Dr. Abeer Mahmoud

43. Multiple-Processor Scheduling
CPU scheduling is more complex when multiple CPUs are available.
load sharing becomes possible
Homogeneous processors -processors are identical in functionality (i.e. any processor can run any process in the ready queue)
Dr. Abeer Mahmoud

44. Multiple-Processor Scheduling
Approaches to Multiple-Processor Scheduling
Asymmetric multiprocessing
Symmetric multiprocessing (SMP)
Master processor executes system code & slave processors execute user code
Only the master processor has all scheduling decisions, I/O processing, and other system activities
Simple & reduce the need for data sharing
Each processor is self-scheduling,
All processes in common ready queue, or each has its own private queue of ready processes
OS must ensure that two processors do not choose the same process and that processes are not lost from the queue.
Dr. Abeer Mahmoud

45. Thank you End of Chapter 5
Dr. Abeer Mahmoud