Process and its related concept with problems.

JIT BarabankiMr Satyendra Mohan Srivastava

1/24/2017 Mr Satyendra Mohan Srivastava, JIT Barabanki 2
Process
 A process is a program in execution
 A process can also be defined as the unit of work in a time sharing system
 A system consist of bunch or collection of processes
 Operating system processes execute system codes and various user
processes executing various user codes
 The various processes can execute concurrently via CPU multiplexing
mechanism
 The CPU switches between various processes which make computer system
more productive

Process (conti..)
 Most of the text books of operating system uses job and processes interchangeably
 Program can be passive entity stored on disk (executable file) but process is active
entity
 Program becomes process when executable file loaded into memory
 A program can be divided into several processes
 A process can be larger than program code (text selection)
 Process can also includes various activities which is represented by the value of the
program counter and contents of processors registers
 A process includes the process stack (temporary data) and a data section (global
variables)

When the process executes, it may changes its state
 new: The process is created in this state
 running: The various instructions are being executed
 waiting: The various process are waiting for some event to occur
 ready: The various process are waiting to be assigned to a processor
 terminated: The execution of process has finished
Process State

Diagrammatic Representation of
Process State
Interrupt
New Terminated
Ready Running
Waiting
Scheduler dispatch
Admitted Exit
I/O or Event waitI/O or Event Completion

Process Control Block (PCB)
Pointer
Process
State
Process Number
Program Counters
Registers
Memory Limits
List of open files
.
.
.
 Each process in the operating system is
represented by a PCB
 PCB is also known as a task control block
 PCB contains pieces of information
associated with a specific process. The
various block along with the description
are listed below:
 Process state : The states like new, ready,
running, waiting, terminated and so on PCB

Process Control Block (PCB)
Pointer
Process
State
Process Number
Program Counters
Registers
Memory Limits
List of open files
.
.
.
 Program Counter : It indicates the next
instruction address which is to be
executed
 Registers : It include accumulators, stack
pointers, index registers, general purpose
registers and any conditional code
information
PCB

CPU Switching from Process to
Process
Process P(i) Operating System Process P(i+1)
Interrupt or system call
Save state into PCB
Save state into PCB
Save state into PCB
Save state into PCB
Executing
Executing
Executing
idle
idle
idle
Interrupt or system call

Threads
 A process is a program that performs a single thread of execution
 A single thread of instructions allows the process to perform one task
at one time
 Various modern operating system have extended the basic process
concept to have multiple thread of execution
 The process can perform more than one task at a time
 A thread is also known as light weight process
 A thread can be define as primary unit of CPU utilization

Threads
Advantages:
 Resource sharing
 Responsiveness
 Economy
 Utilization of multiprocessor architecture

Process Scheduling
 The objective of Multiprogramming is to maximize the number of process
running at all time. This leads to maximize CPU utilization.
 The objective of Time-sharing is to maximize the CPU switching among
the process so that users can interact with each process or program while
it is running
 Only one process is running on uniprocessor system all the time
 If more than one processes exit, the rest process may wait until CPU is
free or scheduled on the CPU
 The various processes in the system are put into job queue
 The job queue contain all the processes in the system

 In Main Memory, the processes are ready and waiting to execute. Such
state in the list are known as the ready queue
 The processes in the list are stored as a linked list format
 A ready queue header in the list contain the pointer which points to the
first and the final Process Control block in the list
 The operating system may also contain other queues
 The various processes are going to provide the CPU then processes may
execute, gets interrupted and wait for a particular event (I/O request)
 The various request of processes can be depicted on the tape drive or on
the disk
Process Scheduling

Process Scheduling
 On the disk, system may be busy with the I/O request of some other
processes hence processes on the disk have to wait for its number to
acquire CPU
 The waiting processes of the list for a particular I/O device are known as
device queue
 Each individual device has its own device queue
 The diagrammatic representation of device which represent process
scheduling is known as queueing diagram
 Two type of Queue are present in the operating system
 The ready queue
 A set of device queue

Process Scheduling
 When the CPU is assigned to the process and process start its execution,
following are the possibilities:
 The process may issue an I/O request and placed on an I/O queue
 The process may create new sub-processes and wait for its completion
 Due to interrupt, the process may removed from CPU and put on the
ready queue

Process Scheduling
Ready Queue CPU
I/O I/O queue I/O request
Time slice expired
Child executes
Interrupt occurs
Fork a child
Wait for an
interrupt

Schedulers
 The operating system select processes from various scheduling queue
 The selection of process is purely dependent on appropriate scheduler
 The processes are spooled to a disk(mass storage device) for some later
execution
 CPU scheduler or Short-term scheduler
 Select next process to be executed and provide the CPU
 Sometimes, the only available scheduler in a particular system
 CPU scheduler is invoked so frequently that it is considered as the fast
one

Schedulers
 Job scheduler or Long-term scheduler
 It select the various processes on the ready queue
 Job scheduler is comparatively less frequently invoked than CPU
scheduler
 It controls the degree of multiprogramming
 Processes can be
CPU-bound process: It spends large time doing computations; few long
CPU bursts
I/O-bound process: It spends large time doing I/O than computations, few
short CPU bursts

Schedulers
Partially executed
swapped-out processes
Ready queue CPU
I/O I/O waiting queues
swap in swap out
In Queuing diagram, addition of Medium Term Scheduler

Context Switch
 Context Switching is the mechanism which allows switching of processes or from
one state to another state
 While CPU switches from one process to another process, the system should save
the state of old process and load the new state (saved state) for the new process
 The Context of a process can be represented via the PCB block diagram
 The system perform no useful task while switching from one process to another.
Such state is known as Context-switch time overhead
 the context switch is directly proportional to the complex OS and the PCB
 Multiple contexts can be loaded at once when multiple sets of registers per CPU
will be provided by the hardware

Operations on Processes
The processes in the system may execute concurrently. They may
create and delete process dynamically. It provide mechanisms
for:
 Process creation
 Process termination
 A process can create new processes with the help of create
process system
 The various creating process is known as a Parent process

 The new created process is known as the Children process
 The newly created process may create various sub processes forming
a tree of processes
 A process need certain resource to complete its task
 Such resources are CPU time, files, memory and I/O devices
 The parent also partition its resource to its children
 Restricting a child process leads to the restriction of process
overloading in the system

 The information or data to the child process is passed via parent
process
 Creation of process leads to two possibilities, Such as
 The child along with the parent continue to execute concurrently
 The parent with the child may wait to complete its execution or
its termination
 Creation of process along with the address space leads to two
possibilities
 Duplication of parent process leads to child process

Producer - Consumer Problem
 A producer process produces data or information that is ultimately
consumed by consumer process
 For concurrent execution of various process, a buffer is required
 A producer process can produce information and consumer process
consume the information produced by consumer process
 A synchronization between producer process and consumer process
is required to overcome various unwanted situation like,
 Consumer may try to consume the information which yet to be
produced. This leads to waiting of consumer process

Producer - Consumer Problem
 Producer may try to produce the information when buffer is full. This
leads to waiting of producer process to consume some information
by consumer process
 The buffer can be provided by the operating system or the application
program
 The buffer can be categorized into two types:
 The Bounded-Buffer
 Assumes unbounded buffer size
 The Unbounded-Buffer
 Assumes a fixed buffer size

Critical Section Problem
 A system consisting of processes has a code segment called a critical section
 At a particular instant of time, Only one process is allowed to perform its
execution in the critical section
 No two processes at any instant of time are allowed in the critical section
 The execution of processes in the Critical Section must be mutually exclusive
 Each process may request permission to enter into critical section
 Critical Section has entry level and exit level section

Critical Section Problem
 A solution to Critical Section problem may satisfy the three requirements:
 Mutual Exclusion
 Progress
 Bounded Waiting
 Mutual Exclusion: When a process reside in the critical section then no two
process is allowed in the critical section
 Progress: The non executing processes in the remainder section can
participate in the decision regarding the next process entry in the critical
section
 Boundary Waiting: There is a maximum limit of request to be made by a
process to enter in the critical section

The Dining Philosophers Problem
Rice

 The five Philosophers are sitting on five chairs sharing a common table
 In the center of the table, some rice in a bowl is placed with the five
chopsticks
 The philosophers may exist in either thinking or eating phase
 In thinking phase, a philosopher doesn’t interact with her colleagues
 When a philosopher is hungry then the philosopher tries to pick up the two
chopsticks that are closer to them
 A philosopher can pick only one chopsticks at a time
 A philosopher is in eating phase when he pick two chopsticks

 After eating, the Philosopher releases two chopsticks
 A philosopher is in eating phase when he pick two chopsticks

Deadlock
 A deadlock is a situation in which waiting process is requesting for the
resources which is hold by another process thus resulting in a situation
known as a Deadlock
 A resource can be memory space, files, CPU cycles and I/O devices
 A resource may be granted to a process when process request for it
 In any situation, a requesting resource can be exceed over the number of
available resources
 In the normal situation, a resource can be granted via following sequence:
 Requesting for resource
 Use of resource
 Release of resource

Deadlock
Note: Release and request for resources are system calls
 A resource can be:
 logical resource
 Physical resource
 Logical Resource
 Semaphores
 Files
 Physical Resource
 Printer
 Memory Space
 CPU cycle
 Tape drives

Deadlock Characterization
 Necessary Conditions: A deadlock may occur if following four conditions
satisfied simultaneously in a system
 Mutual exclusion
 Hold and wait
 No preemption
 Circular wait

Resource Allocation Graph
 A directed graph representation of graph is known as resource
allocation graph
 In resource allocation graph, Following are the representation:
 Process is represented by . in the graph
 Process to resource directed allow head is known as a requesting edge
 Resource to process directed arrow head is known as assignment edge
 A sample resource allocation graph is represented on next slide

A sample Resource Allocation Graph
R1 R3
P1 P2 P3 P4
R2 R4

Deadlock Prevention
Deadlock is the state, we can avoid this state of processes by ensuring that
that atleast one condition can’t hold so by elaborating the necessary four
condition with this approach. The four condition are
 Mutual exclusion
 Hold and wait
 No preemption
 Circular wait

Recovery from Deadlock
 Process termination
 Deadlock elimination via sequential elimination of process
 Elimination of all deadlocked processes
 Resource preemption
 Rollback
 Selecting a victim
 Starvation

Process and its related concept with problems.

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