This a Lecture notes presentation on operating System unit 2.

1. Operating system
Processes , CPU Scheduling And Process
Synchronization

2. Processes
Process Concept, Process Scheduling,
Operation on Processes

3. Process Concept
• A process is an instance of a program in execution.
• Batch systems work in terms of "jobs". Many modern process
concepts are still expressed in terms of jobs, ( e.g. job scheduling ),
and the two terms are often used interchangeably.
• on a single-user system such as Microsoft Windows, a user may be able to run several
programs at one time: a word processor, a web browser, and an e-mail package. Even if
the user can execute only one program at a time, the operating system may need to
support its own internal programmed activities, such as memory management. In many
respects, all these activities are similar, so we call all of them processes.

4. The process
• The text section comprises the compiled program code, read in from non-
volatile storage when the program is launched.
• The data section stores global and static variables, allocated and initialized
prior to executing main.
• The heap is used for dynamic memory allocation, and is managed via calls
to new, delete, malloc, free, etc.
• The stack is used for local variables. Space on the stack is reserved for
local variables when they are declared ( at function entrance or elsewhere,
depending on the language ), and the space is freed up when the variables
go out of scope. Note that the stack is also used for function return values,
and the exact mechanisms of stack management may be language specific.
•Note that the stack and the heap start at opposite ends of the process's free space and grow towards each
other. If they should ever meet, then either a stack overflow error will occur, or else a call to new or malloc
will fail due to insufficient memory available.

5. Process State
As a process executes, it changes state. The state of a process is
defined in part by the current activity of that process. Each process
may be in one of the following states:
 New. The process is being created.
 Running. Instructions are being executed.
 Waiting. The process is waiting for some event to occur (such as an
I/O completion or reception of a signal).
 Ready. The process is waiting to be assigned to a processor.
Terminated. The process has finished execution.

6. Process Scheduling
• The two main objectives of the process scheduling system are to keep
the CPU busy at all times and to deliver "acceptable" response times
for all programs, particularly for interactive ones.
• The process scheduler must meet these objectives by implementing
suitable policies for swapping processes in and out of the CPU.
( Note that these objectives can be conflicting. In particular, every time the system
steps in to swap processes it takes up time on the CPU to do so, which is thereby
"lost" from doing any useful productive work.)

7. Scheduling Queues
• All processes are stored in the job
queue.
• Processes in the Ready state are
placed in the ready queue.
• Processes waiting for a device to
become available or to deliver data
are placed in device queues. There
is generally a separate device queue
for each device.
• Other queues may also be created
and used as needed.

8. Schedulers
• A long-term scheduler is typical of a batch system or a very heavily loaded
system. It runs infrequently, ( such as when one process ends selecting one
more to be loaded in from disk in its place ), and can afford to take the time
to implement intelligent and advanced scheduling algorithms.
• The short-term scheduler, or CPU Scheduler, runs very frequently, on the
order of 100 milliseconds, and must very quickly swap one process out of
the CPU and swap in another one.
• Some systems also employ a medium-term scheduler. When system loads
get high, this scheduler will swap one or more processes out of the ready
queue system for a few seconds, in order to allow smaller faster jobs to
finish up quickly and clear the system

9. Operations on Processes
1.Process Creation
• Processes may create other processes through appropriate system calls, such as fork or spawn. The process
which does the creating is termed the parent of the other process, which is termed its child.
• Each process is given an integer identifier, termed its process identifier, or PID. The parent PID ( PPID ) is
also stored for each process.

10. 2.Process termination
 Processes may request their own termination by making
the exit() system call, typically returning an int. This int is passed
along to the parent if it is doing a wait( ), and is typically zero on
successful completion and some non-zero code in the event of
problems.
 When a process terminates, all of its system resources are freed up,
open files flushed and closed, etc. The process termination status and
execution times are returned to the parent if the parent is waiting for
the child to terminate, or eventually returned to init if the process
becomes an orphan.

11. CPU scheduling
Basic concept ,scheduling criteria
,Scheduling algorithms , Multiple-
Process Scheduling.

12. CPU Scheduling
o The objective of multiprogramming is to have some process running at all times,
to maximize CPU utilization. The idea is relatively simple. A process is executed
until' it must wait, typically for the completion of some I/O request
o In a simple computer system, the CPU then just sits idle. All this waiting time is
wasted; no useful work is accomplished. With multiprogramming, we try to use
this time productively. Several processes are kept in memory at one time. When
one process has to wait, the operating system takes the CPU away from that
process and gives the CPU to another process.
o This pattern continues. Every time one process has to wait, another process can
take over use of the CPU. Scheduling of this kind is a fundamental operating-
system function. Almost all computer resources are scheduled before use. The
CPU is, of course, one of the primary computer resources. Thus, its scheduling is
central to operating-system design.

13. Preemptive Scheduling and non- Preemptive Scheduling
CPU-scheduling decisions may take place under the following four circumstances:
1. When a process switches from the running state to the waiting state (for
example, as the result of an I/O request or an invocation of wait for the
termination of one of the child processes)
2. when a process switches from the running state to the ready state (for
example, when an interrupt occurs)
3. When a process switches from the waiting state to the ready state (for
example, at completion of I/O)
4. When a process terminates

14. When scheduling takes place only under circumstances 1 and 4, we say that the
scheduling scheme is nonpreemptive or cooperative; otherwise, it is preemptive. Under
nonpreemptive 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. This scheduling method was used by Microsoft Windows 3.x; Windows 95
introduced preemptive scheduling, and all subsequent versions of Windows operating
systems have used preemptive scheduling.

15. Scheduling Criteria
Different CPU scheduling algorithms have different properties, and the choice of a particular
algorithm may favor one class of processes over another. In choosing which algorithm to
use in a particular situation, we must consider the properties of the various algorithms.
Many criteria have been suggested for comparing CPU scheduling algorithms. Which
characteristics are used for comparison can make a substantial difference in which algorithm
is judged to be best. The criteria include the following:
CPU utilization. We want to keep the CPU as busy as possible. Conceptually, CPU utilization
can range from 0 to 100 percent. In a real system, it should range from 40 percent (for a lightly
loaded system) to 90 percent (for a heavily used system).
Throughput. If the CPU is busy executing processes, then work is being done. One measure of
work is the number of processes that are completed per time unit, called throughput. For long
processes, this rate may be one process per hour; for short transactions, it may be 10 processes per
second.

16. Turnaround time. From the point of view of a particular process, the important criterion is
how long it takes to execute that process. The interval from the time of submission of a process to
the time of completion is the turnaround time. Turnaround time is the sum of the periods spent
waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O.
 Waiting time. The CPU scheduling algorithm does not affect the amount of time during which
a process executes or does I/O; it affects only the amount of time that a process spends waiting in
the ready queue. Waiting time is the sum of the periods spent waiting in the ready queue.
Response time. In an interactive system, turnaround time may not be the best criterion.
Often, a process can produce some output fairly early and can continue computing new results
while previous results are being output to the user. Thus, another measure is the time from the
submission of a request until the first response is produced. This measure, called response time, is
the time it takes to start responding, not the time it takes to output the response

17. Scheduling Algorithms
There are various algorithms which are used by the Operating
System to schedule the processes on the processor in an
efficient way.
There are the following algorithms which can be used to
schedule the jobs.
1. First Come First Serve
It is the simplest algorithm to implement. The process with the
minimal arrival time will get the CPU first. The lesser the arrival
time, the sooner will the process gets the CPU. It is the non-
preemptive type of scheduling.

18. 2. Round Robin
In the Round Robin scheduling algorithm, the OS defines a time
quantum (slice). All the processes will get executed in the cyclic
way. Each of the process will get the CPU for a small amount of
time (called time quantum) and then get back to the ready queue
to wait for its next turn. It is a preemptive type of scheduling.
3. Shortest Job First
The job with the shortest burst time will get the CPU first. The
lesser the burst time, the sooner will the process get the CPU. It
is the non-preemptive type of scheduling.

19. 4. Shortest remaining time first
It is the preemptive form of SJF. In this algorithm, the OS
schedules the Job according to the remaining time of the
execution.
5. Priority based scheduling
In this algorithm, the priority will be assigned to each of the
processes. The higher the priority, the sooner will the process get
the CPU. If the priority of the two processes is same then they
will be scheduled according to their arrival time.
6. Highest Response Ratio Next
In this scheduling Algorithm, the process with highest response
ratio will be scheduled next. This reduces the starvation in the

20. Multiple-Processor Scheduling
Multiple processor scheduling or multiprocessor scheduling focuses on
designing the system's scheduling function, which consists of more than
one processor. Multiple CPUs share the load (load sharing) in
multiprocessor scheduling so that various processes run simultaneously. In
general, multiprocessor scheduling is complex as compared to single
processor scheduling. In the multiprocessor scheduling, there are many
processors, and they are identical, and we can run any process at any
time.
The multiple CPUs in the system are in close communication, which
shares a common bus, memory, and other peripheral devices. So we can
say that the system is tightly coupled. These systems are used when we
want to process a bulk amount of data, and these systems are mainly used
in satellite, weather forecasting, etc.
Multiprocessor systems may be heterogeneous (different kinds of CPUs)
or homogenous (the same CPU).

21. Approaches to Multiple Processor Scheduling
There are two approaches to multiple processor scheduling in the
operating system: Symmetric Multiprocessing and Asymmetric
Multiprocessing
1.Symmetric Multiprocessing: It is used where each processor
is self-scheduling. All processes may be in a common ready queue,
or each processor may have its private queue for ready processes.
The scheduling proceeds further by having the scheduler for each
processor examine the ready queue and select a process to
execute.
2.Asymmetric Multiprocessing: It is used when all the scheduling
decisions and I/O processing are handled by a single processor
called the Master Server. The other processors execute only
the user code. This is simple and reduces the need for data sharing,
and this entire scenario is called Asymmetric Multiprocessing.

22. Processor Affinity
Processor Affinity means a process has an affinity for the processor
on which it is currently running. When a process runs on a specific
processor, there are certain effects on the cache memory. The data
most recently accessed by the process populate the cache for the
processor. As a result, successive memory access by the process is
often satisfied in the cache memory.
1.Soft Affinity: When an operating system has a policy of keeping 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. Some Linux systems implement soft
affinity and provide system calls like sched_setaffinity() that also
support hard affinity.

23. Load Balancing
• Load Balancing is the phenomenon that keeps the workload evenly
distributed across all processors in an SMP system. Load balancing is
necessary only on systems where each processor has its own private
queue of a process that is eligible to execute.
• Load balancing is unnecessary because it immediately extracts a runnable
process from the common run queue once a processor becomes idle. One
or more processors will sit idle while other processors have high workloads
along with lists of processors awaiting the CPU.
• There are two general approaches to load balancing:
Push Migration: In push migration, a task routinely checks the load on each processor. If it finds an
imbalance, it evenly distributes the load on each processor by moving the processes from
overloaded to idle or less busy processors.
Pull Migration : Pull Migration occurs when an idle processor pulls a waiting task from a busy
processor for its execution

24. Symmetric Multithreading
SMP systems allow several threads to run concurrently by providing multiple physical
processors. An alternative strategy is to provide multiple logical rather than physical-
processors. Such a strategy is known as symmetric multithreading (or SMT); it has also
been termed hyperthreading technology on Intel processors.
The idea behind SMT is to create
multiple logical processors on the same
physical processor, presenting a view of
several logical processors to the
operating system, even on a system
with only a single physical processor.
Each logical processor has its own
architecture state, which includes
general-purpose and machine-state
registers.

25. Process
Synchronization
Background, The Critical-Section
Problems, Synchronization Hardware,
Semaphores , Classical Problem Of
Synchronization.

26. Process Synchronization
o Process Synchronization means coordinating the execution of processes
such that no two processes access the same shared resources and data. It is
required in a multi-process system where multiple processes run together,
and more than one process tries to gain access to the same shared resource
or data at the same time.
o Changes made in one process aren’t reflected when another process
accesses the same shared data. It is necessary that processes are
synchronized with each other as it helps avoid the inconsistency of shared
data.
o several processes access and manipulate the same data concurrently and
the outcome of the execution depends on the particular order in which the
access takes place, is called a race condition
o To prevent race conditions, concurrent processes must be synchronized.

27. The Critical-Section Problem
Consider a system consisting of 11 processes {Po, PI, ...,
Pn-1) Each process has a segment of code, called a
critical section, in which the process may be changing
common variables, updating a table, writing a file, and
so on. The important feature of the system is that, when
one process is executing in its critical section, no other
process is to be allowed to execute in its critical section.
That is, no two processes are executing in their critical
sections at the same time.
The critical-section problem is to design a protocol that
the processes can use to cooperate. Each process must
request permission to enter its critical section. The
section of code implementing this request is the entry
section. The critical section may be followed by an exit
section. The remaining code is the remainder section
The general structure of a typical process Pi is shown in Figure 6.1. The entry section and exit
section are enclosed in boxes to highlight these important segments of code

28. Solution to Critical-Section Problem
1. Mutual Exclusion. If process Pi is executing in its critical section,
then no other processes can be executing in their critical sections.
2. Progress. If no process is executing in its critical section and there
exist some processes that wish to enter their critical section, then
the selection of the processes that will enter the critical section next
cannot be postponed indefinitely.
3. Bounded Waiting. A bound must exist on the number of times
that other processes are allowed to enter their critical sections after
a process has made a request to enter its critical section and before
that request is granted.
• Assume that each process executes at a nonzero speed
• No assumption concerning relative speed of the n processes.

29. Synchronization Hardware
oSynchronization hardware is a hardware-based solution to resolve
the critical section problem. In our earlier content of the critical
section, we have discussed how the multiple processes sharing
common resources must be synchronized to avoid inconsistent
results
oSynchronization hardware i.e. hardware-based solution for the critical
section problem which introduces the hardware instructions that
can be used to resolve the critical section problem effectively.
Hardware solutions are often easier and also improves the efficiency
of the system.
oThe hardware synchronization provides two kinds of hardware
instructions that are TestAndSet and Swap. We will discuss each of
the instruction briefly

30. TestAndSet Hardware Instruction
• The TestAndSet() hardware instruction is atomic instruction. Atomic means
both the test operation and set operation are executed in one machine
cycle at once. If the two different processes are executing TestAndSet()
simultaneously each on different CPU.
• The TestAndSet() instruction can be defined as in the code below:

31. Swap Hardware Instruction
• Like TestAndSet() instruction the swap() hardware instruction is
also an atomic instruction. With a difference that it operates on
two variables provided in its parameter.
• The structure of swap() instruction is:

32. Semaphores
• Semaphores are integer variables that are used to solve the
critical section problem by using two atomic operations, wait
and signal that are used for process synchronization.
• The definitions of wait and signal are as follows −
Wait The wait operation decrements the value of its argument S, if it is
positive. If S is negative or zero, then no operation is performed.

33. Signal The signal operation increments the value of its argument S.
Types of Semaphores
There are two main types of semaphores i.e. counting semaphores and binary
semaphores. Details about these are given as follows −
 Counting Semaphores These are integer value semaphores and have an unrestricted
value domain. These semaphores are used to coordinate the resource access, where the
semaphore count is the number of available resources. If the resources are added,
semaphore count automatically incremented and if the resources are removed, the
count is decremented.
 Binary Semaphores The binary semaphores are like counting semaphores but their
value is restricted to 0 and 1. The wait operation only works when the semaphore is 1
and the signal operation succeeds when semaphore is 0. It is sometimes easier to
implement binary semaphores than counting semaphores.

34. Classical Problems of Synchronization
In this section, present a number of synchronization problems as examples of a large class
of concurrency-control problems. These problems are used for testing nearly every newly
proposed synchronization scheme. In our solutions to the problems, we use semaphores for
synchronization
The classical problems of synchronization are as follows:
1.Bound-Buffer problem
2.Sleeping barber problem
3.Dining Philosophers problem
4.Readers and writers problem

35. Bound-Buffer problem
• Also known as the Producer-Consumer problem. In this problem,
there is a buffer of n slots, and each buffer is capable of
storing one unit of data. There are two processes that are
operating on the buffer – Producer and Consumer. The producer
tries to insert data and the consumer tries to remove data.
• If the processes are run simultaneously they will not yield the
expected output.
• The solution to this problem is creating two semaphores, one full
and the other empty to keep a track of the concurrent processes.
Sleeping Barber Problem
• This problem is based on a hypothetical barbershop with one
barber.
• When there are no customers the barber sleeps in his chair. If any
customer enters he will wake up the barber and sit in the customer
chair. If there are no chairs empty they wait in the waiting

36. Dining Philosopher’s problem
• This problem states that there are K number of philosophers
sitting around a circular table with one chopstick placed between
each pair of philosophers. The philosopher will be able to eat if
he can pick up two chopsticks that are adjacent to the
philosopher.
• This problem deals with the allocation of limited resources.
Readers and Writers Problem
• This problem occurs when many threads of execution try to access
the same shared resources at a time. Some threads may read, and
some may write. In this scenario, we may get faulty outputs.

37. BIBLIOGRAPHY
02124802020 GAURAV KUMAR
02224802020 GAURAV KUMAR
02324802020 HARI OM SINGH
02424802020 HARSHIT GUPTA
02524802020 HARSHIT VASHISHT
02624802020 HEENA SATI
02724802020 HEMANT KUMAR YADAV
02824802020 HIMANSHU
02924802020 KISHAN KUMAR
03024802020 KSHITIZ PAL
03124802020 KUMAR SHIUBHAM
03224802020 KUNAL GUPTA
03324802020 KUNAL SINGH KORANGA
03424802020 KUSHAGRA SINGH
03524802020 LUV KUMAR
03624802020 MOHD AMIR HUSSAIN
03724802020 MOHIT KUMAR
03824802020 MOHIT KUMAR
03924802020 MOHIT KUMAR
04024802020 MUKUL GUPTA
Group 2 student Names who were shown a great contribution in this unit 2 ppt
Sources
www.javatpoint.com
www.geeksforgeeks.org
www.cs.uic.edu
www.binaryterms.com
www.tutorialspoint.com
TEXTBOOK : Silbersachatz and Galvin, “Operating System Concepts”, John Wiley & Sons, 7 th Ed. 2005