Process Description and control

G.Anuradha(Referred from william stallings and

galvin 8th edition)

Contents

• Introduction• Process states• Process description• Process control

Introduction

• OS must interleave the execution of multiple processes to maximize processor utilization

• Proper allocation of resources and avoidance of deadlocks

• Interprocess communications

Process states

• What is it?• A two-state Process model• Creation and termination of processes• Five state model• Suspended processes

What is it?• An operating system executes a variety of programs:

– Batch system – jobs– Time-shared systems – user programs or tasks

• Textbook uses the terms job and process almost interchangeably• Process – a program in execution; process execution must

progress in sequential fashion• A process includes:

– program counter – stack– data section

Trace of the Process

• The behavior of an individual process is shown by listing the sequence of instructions that are executed

• This list is called a Trace• Dispatcher is a small program which switches

the processor from one process to another

Process Execution

• Consider three processes being executed

• All are in memory (plus the dispatcher)

• Lets ignore virtual memory for this.

Trace from the processes point of view:

• Each process runs to completion

Trace from Processors point of view

TimeoutI/O

TimeoutTimeout

A Two-State Process Model

Queuing Diagram

Etc … processes moved by the dispatcher of the OS to the CPU then back to the queue until the task is competed

Review till now

• What is a Trace?• What is a Dispatcher?• What are the two states of a two state process

model?

Creation and Termination of Processes

• Life of a process is bounded by its creation and termination

• When a new process is added– OS builds data structures– Allocates address space in main memory

Process Creation

• The OS builds a data structure to manage the process

• Traditionally, the OS created all processes– But it can be useful to let a running process create

another• This action is called process spawning– Parent Process is the original, creating, process– Child Process is the new process

Reasons for process creation

Process Termination

• There must be some way that a process can indicate completion.

• This indication may be:– A HALT instruction generating an interrupt alert to

the OS.– A user action (e.g. log off, quitting an application)– A fault or error– Parent process terminating

Process Termination

Five State Model

Five State Process Model

• Running: process that is currently being executed• Ready: process that is prepared to execute when

given the opportunity• Blocked: process that is waiting for some event to

occur • New: process that has not yet been loaded into

main memory• Exit: process that is released because its halted

or its aborted

Process model contd…

• In New state a process is defined which happens in 2 stages– OS gives a Id– Tables needed are allocated and built

• While in new state the program remains in secondary storage

Process model contd…

• Termination moves the process to exit state• Exit also occurs in two stages– The tables and other information associated with

job are Temporarily preserved by OS– Once the utility programs extracts the needed

information the OS no longer maintains any data and the process is deleted from the system.

Events that lead to state transition• NullNew: new process is created• Newready : Admit – When OS is prepared to take an additional

process• ReadyRunning: Dispatch When OS has to select a process it selects

one from the ready state• Running exit : Release – When process is completed or is aborted• RunningReady: Time-out or when the process is preempted• Running Blocked: EventWait – Waiting for I/O or any other system

call which is performed by the OS• BlockedReady :Event Occurs• ReadyExit: In some systems a parent may terminate a child process at

any time(not shown in diagram)• Blocked exit:

Single blocked queue diagram

Multiple blocked queues

Suspended Processes

• What is it?• Need for Swapping• Other uses of suspension

Need for Swapping

• The I/O takes more time than processor execution• Even with multiprogramming with lots of I/O

operations the execution speed of processor comes down

• This can be avoided by expanding the main memory. But– Cost of main memory is more – Larger memory results in larger processes not more

processes• Second solution is Swapping

Swapping

• Swapping is moving part or all of a process from main memory to disk

• This happens when none of the processes in main memory are in Ready state

• In the disk the processes are in a suspend queue – which has existing processes that has been kicked out of main memory

• After this the OS brings some other process in the suspend queue or some new process

• With swapping we have an additional state-suspend state

State transition diagram with single suspend state

Need for suspend states

• When swapping takes place the OS can decide to get the process from the new state or from suspend state. This can include more states

• Ready: Process in main memory and available for execution

• Blocked: Process in main memory and awaiting an event• Blocked/Suspend: Process in secondary memory and

awaiting an event• Ready/Suspend: Process in secondary memory but is

available for execution

Process Description

• What is it?• OS control Structures

– Memory tables– I/O tables– File tables– Process tables

• Process Control Structures– Process Location– Process Attributes– Processor state information– Process Control Block

What is Process Description?OS is an entity that manages the use of system resources by processes

What information does the OS need to control processes and manage resources for them?Current status of each process and resourceHow the OS does it? Maintains tables of each entity it’s managing

General Structure of OS control tables

Process Control Structures

• For managing and control a process the OS should know– Where the process is located– Attributes of the process that are necessary for its

management (Process ID, Process State, Location in Memory)

Process Image

Process Location

• Location of a process image will depend on the memory management scheme

• Process image is maintained as a contiguous or continuous block of memory

• This block is maintained in the disk and OS needs to know the location in the disk

• For OS to manage the process at least a small portion of its image must be maintained in main memory

• Modern OS uses memory management schemes like paging or segmentation for bringing only a portion of process image in the main memory

• These details should also be stored in the process table

Process Control Block

• Information about a process is stored in a Process control Block(PCB)

• PCB information is grouped into 3 heads– Process Identification– Process state information– Process Control information

Process Identification

• Identifier of this process• Identifier of the process that created this

process (parent Process)• User Identifier

Processor State Information

Process Control Information

Role of PCB

• It is the most important data structure in an OS• Each PCB contains all of the information about a

process that is needed by the OS• The blocks are read and/or modified by every

module in the OS including– Scheduling– Resource allocation– Interrupt processing– Performance monitoring and analysis

Role of PCB Contd…

• Direct access to PCB acts as a threat to its protection– A bug in a single routine could damage PCB which

could destroy the ability to manage other processes

– A design change in PCB could affect a number of modules in the OS

• Solution is to use a handler routine to protect the PCB

Process Control

• Modes of execution:-– User mode-less privileged mode– Kernel mode- more privileged mode

• Reasons for two modes– Protect the OS, and OS tables like PCB from interference by

user programs• In the kernel mode, the software has complete control of

the processor and its instructions, registers, and memory• Program Status Word (PSW) indicates the mode of

execution

Steps in Process Creation1. Assign a unique PID to the new process:- entry is made to the

primary process table2. Allocate space for the process:-all elements in the process

image like, user address space, user stack are to be set3. Initialize PCB

1. Process ID portion will contain the IDs2. Processor State Information portion will be initialized to

zero(mostly)3. Process Control Information is initialized based on standard default

values plus attributes requested for this process

4. Set Appropriate linkages5. Create or expand other data structures

Process Control Block (PCB)Information associated with each process• Process state• Program counter: Address of the next instruction to be

executed• CPU registers: State information which must be saved

when an interrupt occurs.• CPU scheduling information: process priority, pointers to

scheduling queues, and other scheduling parameters. • Memory-management information: base and limit

registers, base table and segment table• Accounting information: time limits, account and process

numbers etc• I/O status information: list of I/O devices allocated to the

process a list of open files and so on

Process Control Block (PCB)

CPU Switch From Process to Process

QuizAcross2.complete control of processor6.less priviledged mode7.one process creating another8.List of Instructions

Down1.process waiting for input output operation3.Program to switch processor from one process to another process4.moving part or all process from main memory to disk5.Program in execution

Quiz answers