Principle Of Operating System Chapter 2

Unit 2: Process Concept

Prof. Mrs. Trupti S. IndiWalchand Institute of TechnologyDepartment of Information TechnologySolapur University

Prof. T. S. Indi, Walchand Institute of Technology, Solapur University

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Processes

• Process Concept• Process Scheduling• Operations on Processes• Interprocess Communication (IPC)• Threads


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Process Concept

• An OS executes a variety of programs:

- Batch system: jobs

- Time- sharing systems: user programs or tasks

• Process is a program in execution

• A process is the unit of work in a modern time-sharing system

• A process is more than program code, it also includes current

activity

• A process includes: PC, stack and data section


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Process In Memory


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Process Concept

• A process includes: PC, stack and data section• The process stack contains:– Temporary data such as method parameters

return addresses and local variables• A data section contains global variables

• “A program by itself is not a process”– Program is a passive entity such as file on disk– Process is an active entity, with PC specifying _?__

and set of associated resources


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Process State

As a process executes, it changes state1) new: The process is being created2) running: Instructions are being executed3) waiting: The process is waiting for some

event to occur4) ready: The process is waiting to be assigned

to a processor5) terminated: The process has finished

execution


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Diagram of Process States

1. Admitted2. Interrupted3. Scheduler Dispatch4. I/O or event wait5. I/O or event completion6. Exit


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Diagram of Process States

• Only one process can be running on any processor at any instant.

• Many processes many be ready and waiting.


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Process Control Block (PCB)

Each process is represented by PCB also called task control block

i. Process stateii. Program counteriii. CPU registersiv. CPU scheduling informationv. Memory-management informationvi. Accounting informationvii. I/O status information


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Process Control Block (PCB)PCB contains information associated with a specific process given as:

Process state: The state may be new, ready, running, waiting, and terminated.

Program Counter: The counter indicates the address of the next instruction to be executed for this process.

CPU registers: The registers vary in number and type, depending on the computer architecture.They include accumulators, index registers, stack pointers and GPRs, condition code info.


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QUIZ1) Process is ____________________________2) Program is ______ entity and process is ________ entity.

(active , passive)3) Process states are ___, _____, _____, _____, and ______4) Process includes ____, _______ and ______5) In which state process is, if “Process is waiting to be

assigned to a processor”6) PCB is stands for __________________ and it is also called

as ____________________7) When I/O or event completion occurs then process

changes its state from ______ to ______8) How many process can be in running at any instant on

single processor ?


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CPU Switch From Process to Process


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• When current executing process P0 interrupted, the OS saves the sate into PCB0 of P0 and load the PCB1 of process P1 and P0 process will be idle and P1 will be executing.

• After P1 execution is completed, the OS will save state into PCB1 and reload state from PCB0 and P1 process will be idle whereas P0 will be executing.

CPU Switch From Process to Process


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• When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process

• Context-switch time is overhead; the system does no useful work while switching

• Time dependent on hardware support

Context Switch


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• Multiprogramming maximizes ___________ ?

• Time-sharing switches CPU among _________ and improves user _____________

• For uni-processor system if one process is running the rest one are waiting until the CPU is free and can be _____________

Process Scheduling


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We define process scheduling as:• The act of determining which process in the

ready state should be moved to the running state.

• That is, decide which process should run on the CPU next.

Process Scheduling

http://www.sal.ksu.edu/faculty/tim/ossg/glossary.html


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Process Scheduling Queue

• Job queue – set of all processes in the system

• Ready queue – set of all processes residing in main memory, ready and waiting to execute

• Device queues – set of processes waiting for an I/O device

• Processes migrate among the various queues


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Ready Queue and Various I/O Device Queues


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Representation of Process Scheduling


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The goal of the scheduler:• Implement the virtual machine in such a

way the user perceives that each process is running on it’s own computer.

Schedulers


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• Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue

• Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU

Schedulers


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• Short-term scheduler is invoked very frequently (milliseconds) (must be fast)

• Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow)

• The long-term scheduler controls the degree of multiprogramming

• Processes can be described as either:– I/O-bound process – spends more time doing I/O than

computations, many short CPU bursts

– CPU-bound process – spends more time doing computations; few very long CPU bursts

Schedulers


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I/O Bound Process:• Processes that are mostly waiting for the completion of

input or output (I/O) are I/O Bound.

• Interactive processes, such as office applications are mostly I/O bound the entire life of the process. Some processes may be I/O bound for only a few short periods of time.

• The expected short run time of I/O bound processes means that they will not stay the running the process for very long.

• They should be given high priority by the scheduler.

Schedulers


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SchedulersCPU Bound Process:• CPU Bound processes are ones that are implementing algorithms

with a large number of calculations.

• They can be expected to hold the CPU for as long as the scheduler will allow.

• Programs such as simulations may be CPU bound for most of the life of the process.

• Users do not typically expect an immediate response from the computer when running CPU bound programs.

• They should be given a lower priority by the scheduler.


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• Parent process create children processes, which, in turn create other processes, forming a tree of processes

• Resource sharing– Parent and children share all resources– Children share subset of parent’s resources– Parent and child share no resources

• Execution– Parent and children execute concurrently– Parent waits until children terminate

Process Creation


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• Address space– Child duplicate of parent– Child has a program loaded into it

• UNIX examples– fork system call creates new process– exec system call used after a fork to replace the

process’ memory space with a new program

Process Creation


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Process Creation


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#include<unistd.h>#include<sys/types.h>#include<stdio.h>#include<stdlib.h>

int main(){

pid_t pid;/* fork another process */

pid = fork();if (pid < 0) { /* error

occurred */fprintf(stderr, "Fork

Failed");exit(-1);

}

C Program Forking Separate Processelse if (pid == 0) { /* child

process */execlp("/bin/ls", "ls", NULL);

}else { /* parent process *//* parent will wait for the child to complete */wait (NULL);printf ("Child Complete");

exit(0);}

}


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Output

[tsindi@localhost ~]$ vi NewProc.c[tsindi@localhost ~]$ cc NewProc.c[tsindi@localhost ~]$ ./a.outa.out F1.cpp f2Exe GetInfo.cpp NonRecursive.cass1.c f1Output fcfsPgm.cpp getInfoExe PolyLL.cBinaryTree.c F2.cpp forkPgm.c NewProc.cChild Complete[tsindi@localhost ~]$ vi NewProc.c


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Linux/Unix Command exec:DESCRIPTION• This command treats its arguments as the

specification of one or more subprocesses to execute. The arguments take the form of a standard shell pipeline where each arg becomes one word of a command, and each distinct command becomes a subprocess.

• If the initial arguments to exec start with - then they are treated as command-line switches and are not part of the pipeline specification.

exec Family of Functions


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A tree of processes on a typical Solaris


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Process Termination• Process executes last statement and asks the

operating system to delete it (exit)– Output data from child to parent (via wait)– Process’ resources are deallocated by operating system

• Parent may terminate execution of children processes (abort)– Child has exceeded allocated resources– Task assigned to child is no longer required– If parent is exiting• Some operating system do not allow child to continue if its

parent terminates– All children terminated - cascading termination


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Study

• fork()• wait()• exec() family of functions execl(), execlp(),

execle(), execv(), execvp(), execvpe()• sleep()• exit()• abort()


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• Independent process cannot affect or be affected by the execution of another process

• Cooperating process can affect or be affected by the execution of another process

• Advantages of process cooperation– Information sharing – Computation speed-up– Modularity– Convenience

Cooperating Processes


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• Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process– unbounded-buffer places no practical limit on the

size of the buffer– bounded-buffer assumes that there is a fixed

buffer size

Producer-Consumer Problem


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Continue…

Software

Principle Of Operating System Chapter 2