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CSE325 Principles of Operating Systems

Processes

David P. Duggan

[email protected]

February 1, 2011


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Todays Goal:

1. Process Concept

2. Process Manager Responsibilities

3. Process Scheduling

4. Operations on Processes

5. Cooperating Processes

6. Inter-process Communication


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Process Concept

Early systems allowed one program at a time

Poor utilization of expensive hardware

Defn: A Process is an abstraction of a processor

Process is also a program in execution

Tighter control and compartmentalization

Unit of work in modern time-sharing systems

Modern systems consist of a collection of processes

A program is static, a process is dynamic



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Basic Abstractions

Program ResultAbstract

Machine

Physical

Machine

Abstract

MachineProgram Result

Idea

Idea

Abstract

MachineProgram Result

Idea


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How Multiprogramming Works

Processor

Process 1

Process 2

Process 3

Process 4

Space-multiplexed Memory

Time-multiplexed CPU


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Process Abstraction

Operating System

Hardware

Data

Process

S

tack

Processor

Executable

Memory

Program


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OSAddress

Space

Implementing the Process Abstraction

Control

Unit

OS interface

Mac

hineExecutableM

emory

ALU

CPU

Pi AddressSpace

Pi CPU

Pi Executable

Memory

PkAddress

Space

Pk CPU

Pk Executable

Memory

PjAddress

Space

Pj CPU

Pj Executable

Memory

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Formal Model

The process concept is used to model:

The states of a system state

Some sequence of status values computation

Means of generating a class of such sequences actionfunction

State-variables

Elementary quantities

Can assume well-defined values

State

Mapping from state-variables into values



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Formal Model, contd

State Space

Set of possible states for a given set of state-variables

Computation

Sequence of states, starting with the initial state

Action Function

Mapping from states into states

Process is a triple (S,f,I), where S: state space

f: action function in that state space

I


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Formal Model, contd

A process generates all computations generated by its action

function from its initial states

Intuitively

the variable set is memory

a state gives the contents of memory

the action function is a program

If two processes have disjoint state variable sets, they cannot

interact

Communication among processes is modeled by shared variables

Multiple processes may share WHAT, acting on different variable

sets

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External View of the Process Manager

Hardware

Application

Program

DeviceMgr

P

rocessMgr

M

emoryMgr

FileMgr

UNIX

DeviceMgr

P

rocessMgr

M

emoryMgr

FileMgr

Windows

CreateThread()CreateProcess()

CloseHandle()

WaitForSingleObject()

fork()

exec()wait()

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Process Manager Overview

Program Process

Abstract Computing Environment

File

Manager

MemoryManager

DeviceManager

Protection

Deadlock

Synchronization

Process

Description

CPU Other H/W

SchedulerResource

ManagerResource

ManagerResource

Manager

MemoryDevices


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Process Concept

An operating system executes a variety of programs:

Batch system jobs

Time-shared systems user programs or tasks

We use the termsjoband processalmost interchangeably.

Process a program in execution; process execution must

progress in sequential fashion.

A process includes:

program counter

stack

data section


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Algorithms, Programs, and Processes

Data

FilesFiles

Files

Other

Resources

Idea

Algorithm

Source

ProgramBinary

Program

Execution Engine

Process

StackStatus

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Process State

As a process executes, it changes state

new: The process is being created.

ready: The process is waiting to be assigned to a

processor.

running: Instructions are being executed.

waiting: The process is waiting for some event to

occur. terminated: The process has finished execution.


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Diagram of Process State

Program


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UNIX State Transition Diagram

Runnable

StartAllocate

Done

Zombie

Wait by

parentRunning

Schedule

Request

Sleeping

Request

Uninterruptible

Sleep

I/O Request

I/O Complete

Traced or Stopped

Resume


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Windows NT Thread States

Initialized

CreateThread

Ready

Activate

Select

Standby

Running

Terminated

Waiting

Transition

Reinitialize

Exit

Preem

pt

Dispatch

Wait

Wait Complete

Wait Complete

Dispatch


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Address Space

Process must be able to reference every resource in

its abstract machine

Assign each unit of resource an address

Most addresses are for memory locations

Abstract device registers

Mechanisms to manipulate resources

Addresses used by one process are inaccessible toother processes

Say that each process has its own address space


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Process & Address Space

Address Space

Code Resources

Resources

Abstract Machine Environment

Stack

Data

Resources

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The Address Space

Process

Address

Space

Address

Binding

Executable

Memory

Other objects

Files

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Process Control Blocks (PCB)or Process Descriptors

Information (data structure) associated with

each process.

Process state

Program counter

CPU registers

CPU scheduling information

Memory-management information

Accounting information

I/O status information


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Process Control Block (PCB)

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EPROCESS

voi d *Uni quePr ocessI d;

NT Executive

Windows NT Process Descriptor

KPROCESS

ui nt 32 Ker nel Ti me;ui nt 32 User Ti me;Byt e st at e;

NT Kernel


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CPU Switch From Process to Process


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Context Switching

Process

Manager

Interrupt

Handler

P1

P2

Pn

Executable Memory

Initialization 1

23

4

5

7Interrupt

8

9

6

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Context Switch

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.

How to reduce context-switch overheads?

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