Operating System Sheduling

B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar
Operating System
KCS – 401
CPU Scheduling
Dr. Pankaj Kumar
Associate Professor – CSE
SRMGPC Lucknow

Outline of the Lecture
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Real-Time CPU Scheduling
Operating Systems Examples
Algorithm Evaluation

Basic Concept
Maximum CPU utilization obtained with multiprogramming
CPU–I/O Burst Cycle – Process execution consists of a cycle of
CPU execution and I/O wait
CPU burst followed by I/O burst
CPU burst distribution is of main concern
Burst Time/Execution Time: It is a time required by the process to
complete execution. It is also called running time.
Arrival Time: when a process enters in a ready state.

CPU Scheduling
CPU Scheduling is a process of determining which process will own CPU for execution while
another process is on hold.
The main task of CPU scheduling is to make sure that whenever the CPU remains idle, the OS at
least select one of the processes available in the ready queue for execution.
The selection process will be carried out by the CPU scheduler. It selects one of the processes in
memory that are ready for execution.

CPU Scheduling
Types of CPU Scheduling
Preemptive Scheduling
In Preemptive Scheduling, the tasks are mostly
assigned with their priorities. Sometimes it is
important to run a task with a higher priority
before another lower priority task, even if the
lower priority task is still running. The lower
priority task holds for some time and resumes
when the higher priority task finishes its
execution.
Non-Preemptive Scheduling
In this type of scheduling method, the CPU has
been allocated to a specific process. The process
that keeps the CPU busy will release the CPU
either by switching context or terminating. It is
the only method that can be used for various
hardware platforms. That's because it doesn't
need special hardware (for example, a timer) like
preemptive scheduling.

CPU Scheduler
Short-term scheduler selects from among the processes in ready queue, and allocates the
CPU to one of them
Queue may be ordered in various ways
CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
Scheduling under 1 and 4 is nonpreemptive
All other scheduling is preemptive
Consider access to shared data
Consider preemption while in kernel mode
Consider interrupts occurring during crucial OS activities

Dispatcher
Dispatcher module gives control of the CPU to the process selected by the short-term scheduler;
this involves:
switching context
switching to user mode
jumping to the proper location in the user program to restart that program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running

Scheduling Criteria
CPU utilization: CPU utilization is the main task in which the
operating system needs to make sure that CPU remains as busy as
possible. It can range from 0 to 100 percent.
Throughput: The number of processes that finish their execution
per unit time is known Throughput. So, when the CPU is busy
executing the process, at that time, work is being done, and the
work completed per unit time is called Throughput.
Waiting time: Waiting time is an amount that specific process needs to wait in the ready queue.
Response time: It is an amount to time in which the request was submitted until the first
response is produced.
Turnaround Time: Turnaround time is an amount of time to execute a specific process. It is the
calculation of the total time spent waiting to get into the memory, waiting in the queue and,
executing on the CPU. The period between the time of process submission to the completion time
is the turnaround time.

Scheduling Criteria
Completion Time: Time at which process completes its
execution.
Turn Around Time: Time Difference between
completion time and arrival time.
Turn Around Time = Completion Time – Arrival Time
Waiting Time(W.T): Time Difference between turn
around time and burst time.
Waiting Time = Turn Around Time – Burst Time
Waiting time = Start time - Arrival time

Scheduling Type
Types of CPU scheduling Algorithm
There are mainly six types of process scheduling algorithms
1. First Come First Serve (FCFS)
2. Shortest-Job-First (SJF) Scheduling
3. Shortest Remaining Time
4. Priority Scheduling
5. Round Robin Scheduling
6. Multilevel Queue Scheduling

First Come First Serve (FCFS)
It is the easiest and most simple CPU scheduling algorithm. In this type of algorithm, the process
which requests the CPU gets the CPU allocation first. This scheduling method can be managed
with a FIFO queue.
As the process enters the ready queue, its PCB (Process Control Block) is linked with the tail of
the queue. So, when CPU becomes free, it should be assigned to the process at the beginning of
the queue.
Characteristics of FCFS method:
• It offers non-preemptive and pre-emptive scheduling algorithm.
• Jobs are always executed on a first-come, first-serve basis
• It is easy to implement and use.
• However, this method is poor in performance, and the general wait time is quite high.

Process Burst Time
P1 24
P2 3
P3 3
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: (0 + 24 + 27)/3 = 17
P P P
1 2 3
0 24 30
27

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: (6 + 0 + 3)/3 = 3
Much better than previous case
P1
0 3 6 30
P2
P3
Convoy effect - short process behind long process
Consider one CPU-bound and many I/O-bound processes

Process Burst time Arrival time
P1 6 2
P2 3 5
P3 8 1
P4 3 0
P5 4 4
Waiting time for P1 = 11-2 = 9
Waiting time for P4 = 0
Average waiting time: (9 + 16 + 2+0+13)/5 = 40/5 =8
Turn Around Time = P1 = 17-2 = 15
Average Turn Around Time : (15 + 18 + 10+3+17)/5 = 63/5 =12.6

Process Id Arrival time Burst time
P1 3 4
P2 5 3
P3 0 2
P4 5 1
P5 4 3
Average Turn Around time = (4 + 8 + 2 + 9 + 6) / 5 = 29 / 5 = 5.8
Waiting time for P1 = 3 – 3 = 0
Waiting time for P3 = 0
Average waiting time: (0 + 5 + 0 + 8 + 3)/5 = 16/5 =3.2

Shortest Job First Serve (SJFS)
1. It is a Greedy Algorithm.
2. Sort all the process according to the arrival time.
3. Then select that process which has minimum arrival time and minimum Burst time.
4. After completion of process make a pool of process which after till the completion of
previous process and select that process among the pool which is having minimum Burst
time.
• It may cause starvation if shorter processes keep coming. This problem can be solved using
the concept of ageing.
• It is practically infeasible as Operating System may not know burst time and therefore may
not sort them. While it is not possible to predict execution time, several methods can be used
to estimate the execution time for a job, such as a weighted average of previous execution
times. SJF can be used in specialized environments where accurate estimates of running time
are available.

Process Burst Time
P1 6
P2 8
P3 7
P4 3
SJF scheduling chart
Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
P3
0 3 24
P4
P1
16
9
P2

Now we add the concepts of varying arrival times and preemption to the
analysis
Process Arrival Time Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
Preemptive SJF Gantt Chart
Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec
Preemptive mode of Shortest Job First is called as Shortest Remaining Time First (SRTF).

Process Id Arrival time Burst time
P1 3 1
P2 1 4
P3 4 2
P4 0 6
P5 2 3
set of 5 processes whose arrival time and burst
time are given below-
•Average Turn Around time = (4 + 15 + 5 + 6 + 10) / 5 = 40 / 5 = 8 unit
•Average waiting time = (3 + 11 + 3 + 0 + 7) / 5 = 24 / 5 = 4.8 unit

Advantages-
• SJFS is optimal and guarantees the minimum average waiting time.
• It provides a standard for other algorithms since no other algorithm performs better than it.
Disadvantages-
• It can not be implemented practically since burst time of the processes can not be known in
advance.
• It leads to starvation for processes with larger burst time.
• Priorities can not be set for the processes.
• Processes with larger burst time have poor response time.

Priority Scheduling
Priority Scheduling is a method of scheduling processes that is based on priority. In this
algorithm, the scheduler selects the tasks to work as per the priority.
The processes with higher priority should be carried out first, whereas jobs with equal priorities
are carried out on a round-robin or FCFS basis. Priority depends upon memory requirements,
time requirements, etc.
Characteristics of Priority Scheduling
• It used in Operating systems for performing batch processes.
• If two jobs having the same priority are READY, it works on a FIRST COME, FIRST
SERVED basis.
• In priority scheduling, a number is assigned to each process that indicates its priority level.
• Lower the number, higher is the priority.
• In this type of scheduling algorithm, if a newer process arrives, that is having a higher priority
than the currently running process, then the currently running process is preempted.

Priority Scheduling
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
Average waiting time = (6+0+16+18+1)/5 = 8.2 msec

Priority Scheduling
Process Priority Burst time Arrival time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
P5 2 2 12
Average Waiting time = (0+11+0+5+2)/5 = 18/5= 3.6

Priority Scheduling
Advantages
Processes are executed on the basis of priority so high priority does not need to wait for long
which saves time
Suitable for applications with fluctuating time and resource requirements.
Disadvantages of priority scheduling
Starvation – low priority processes may never execute
Solution  Aging – as time progresses increase the priority of the process

Round-Robin Scheduling
• Round robin is a pre-emptive algorithm
• The CPU is shifted to the next process after fixed interval time, which is called time
quantum/time slice.
• The process that is preempted is added to the end of the queue.
• Round robin is a hybrid model which is clock-driven
• Time slice should be minimum, which is assigned for a specific task that needs to be
processed. However, it may differ OS to OS.
• It is a real time algorithm which responds to the event within a specific time limit.
• Round robin is one of the oldest, fairest, and easiest algorithm.
• Widely used scheduling method in traditional OS.

Process Queue Burst time
P1 4
P2 3
P3 5
quantum/time slice = 2
Waiting Time
P1= 8-4= 4
P2= 9-3= 6
P3= 12-5= 7
Turnaround Time
P1= 8
P2= 9
P3= 12
Average Waiting time = (4+6+7)/3 = 5.66

Process ID Arrival Time Burst Time
1 0 5
2 1 6
3 2 3
4 3 1
5 4 5
6 6 4
Process
ID
Completion
Time
Turn
Around
Time
Waiting
Time
1 17 17 12
2 23 22 16
3 11 9 6
4 12 9 8
5 24 20 15
6 21 15 11
Avg Waiting Time = (12+16+6+8+15+11)/6 = 76/6 units
P1-P2-P3-P4-P5-P1-P6-P2-P5

Disadvantages
• If slicing time of OS is low, the processor output will be reduced.
• This method spends more time on context switching
• Its performance heavily depends on time quantum.
• Priorities cannot be set for the processes.
• Round-robin scheduling doesn't give special priority to more important tasks.
• Lower time quantum results in higher the context switching overhead in the system.
• Finding a correct time quantum is a quite difficult task in this system.

Multilevel Queue Scheduling
Multilevel queue scheduling algorithm partitions the ready
queue into several separate queues.
A common division is made between foreground (or
interactive) processes and background (or batch) processes.
These two types of processes have different response-time
requirements, and so might have different scheduling needs. In
addition, foreground processes may have priority over
background processes.
Multilevel queue scheduling has the following characteristics:
• Each queue has its own scheduling algorithm
• Processes are divided into different queues based on their
process type, memory size and process priority.

Multilevel Queue Scheduling
• System Process The Operating system itself has its own process to
run and is termed as System Process.
• Interactive Process The Interactive Process is a process in which
there should be the same kind of interaction (e.g. online game ).
• Batch Processes Batch processing is basically a technique in the
Operating system that collects the programs and data together in
the form of the batch before the processing starts.
• Student Process The system process always gets the highest
priority while the student processes always get the lowest priority.
For System Processes: First Come First Serve(FCFS) Scheduling.
For Interactive Processes: Shortest Job First (SJF) Scheduling.
For Batch Processes: Round Robin(RR) Scheduling
For Student Processes: Priority Scheduling

Multilevel Feedback Queue Scheduling
This Scheduling is like Multilevel Queue (MLQ) Scheduling but in this
process can move between the queues. Multilevel Feedback Queue
Scheduling (MLFQ) keep analyzing the behavior (time of execution) of
processes and according to which it changes its priority.
Three queues:
Q1 – RR with time quantum 8 milliseconds
Q2 – RR time quantum 16 milliseconds
Q3 – FCFS
Scheduling
A new job enters queue Q0 which is served FCFS
When it gains CPU, job receives 8 milliseconds
If it does not finish in 8 milliseconds, job is moved to queue Q1
At Q1 job is again served FCFS and receives 16 additional
milliseconds
If it still does not complete, it is preempted and moved to
queue Q2

Multiprocessor Scheduling
Multiple processor scheduling or multiprocessor scheduling focuses on designing the scheduling
function for the system which is consist of ‘more than one processor’. With multiple processors
in the system, the load sharing becomes feasible but it makes scheduling more complex.
CPU scheduling more complex when multiple CPUs are available
Homogeneous processors within a multiprocessor
Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating
the need for data sharing
Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common
ready queue, or each has its own private queue of ready processes

Multiprocessor Scheduling
Processor affinity – process has affinity for processor on which it is currently running
1. Soft Affinity – When an operating system has a policy of attempting to keep a process
running on the same processor but not guaranteeing it will do so, this situation is called soft
affinity.
2. Hard Affinity – Hard Affinity allows a process to specify a subset of processors on which it
may run.
If SMP, need to keep all CPUs loaded for efficiency
Load balancing attempts to keep workload evenly distributed
Push migration – periodic task checks load on each processor, and if found pushes task from
overloaded CPU to other CPUs
Pull migration – idle processors pulls waiting task from busy processor

Operating System Sheduling

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