CSE325 Principles of Operating Systems

Processes

David P. Duggan

dduggan@sandia.gov

January 29, 2013

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Today’s Goal:

1.  Process Concept

2.  Process Manager Responsibilities

3.  Process Scheduling

4.  Operations on Processes

5.  Cooperating Processes

6.  Inter-process Communication

Process Concept

n  Early systems allowed one program at a time n  Poor utilization of expensive hardware

n  Defn: A Process is an abstraction of a processor n  Process is also a program in execution

n  Tighter control and compartmentalization

n  Unit of work in modern time-sharing systems

n  Modern systems consist of a collection of processes

n  A program is static, a process is dynamic

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4

Basic Abstractions

Program Result Abstract Machine

Physical Machine

Abstract Machine Program Result

Idea

Idea

Abstract Machine Program 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

Stac

k

Processor

Executable Memory

Program

7

OS Address Space

Implementing the Process Abstraction

Control Unit

OS interface

…

Mac

hine

Exe

cuta

ble

Mem

ory

ALU

CPU Pi Address

Space

Pi CPU

Pi Executable Memory

Pk Address Space

…

Pk CPU

Pk Executable Memory

Pj Address Space

Pj CPU

Pj Executable Memory

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Formal Model

n  The process concept is used to model: n  The states of a system – state

n  Some sequence of status values – computation

n  Means of generating a class of such sequences – action function

n  State-variables n  Elementary quantities

n  Can assume well-defined values

n  State n  Mapping from state-variables into values

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Formal Model, contd

n  State Space n  Set of possible states for a given set of state-variables

n  Computation n  Sequence of states, starting with the initial state

n  Action Function n  Mapping from states into states

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

n  f: action function in that state space

n  I <= S: set of initial states

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Formal Model, contd

n  A process generates all computations generated by its action function from its initial states

n  Intuitively n  the variable set is memory

n  a state gives the contents of memory

n  the action function is a program

n  If two processes have disjoint state variable sets, they cannot interact n  Communication among processes is modeled by shared variables

n  Multiple processes may share , acting on different variable sets

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External View of the Process Manager

Hardware

Application Program

Dev

ice

Mgr

Proc

ess M

gr

Mem

ory

Mgr

Fi

le M

gr

UNIX

Dev

ice

Mgr

Proc

ess M

gr

Mem

ory

Mgr

Fi

le M

gr

Windows

CreateThread() CreateProcess() CloseHandle()

WaitForSingleObject()

fork()

exec() wait()

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Process Manager Overview

Program Process

Abstract Computing Environment

File Manager

Memory Manager

Device Manager

Protection Deadlock

Synchronization

Process Description

CPU Other H/W

Scheduler Resource Manager Resource Manager Resource Manager

Memory Devices

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Process Concept

n  An operating system executes a variety of programs: n  Batch system – jobs

n  Time-shared systems – user programs or tasks

n  We use the terms job and process almost interchangeably.

n  Process – a program in execution; process execution must progress in sequential fashion.

n  A process includes: n  program counter

n  stack

n  data section

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Algorithms, Programs, and Processes

Data

Files Files Files

Other Resources

Idea

Algorithm

Source Program

Binary Program

Execution Engine

Process

Stack Status

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Process State

n  As a process executes, it changes state n  new: The process is being created.

n  ready: The process is waiting to be assigned to a processor.

n  running: Instructions are being executed.

n  waiting: The process is waiting for some event to occur.

n  terminated: The process has finished execution.

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Diagram of Process State

Program

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UNIX State Transition Diagram

Runnable

Start Allocate

Done

Zombie

Wait by parent

Running

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

Sele

ct

Standby

Running

Terminated

Waiting

Transition

Reinitialize

Exit

Pree

mpt

Dispatch

Wait Wait Complete

Wait Complete

Dispatch

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Address Space

n  Process must be able to reference every resource in its abstract machine

n  Assign each unit of resource an address

n  Most addresses are for memory locations

n  Abstract device registers

n  Mechanisms to manipulate resources

n  Addresses used by one process are inaccessible to other processes

n  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

n  Information (data structure) associated with each process. n  Process state

n  Program counter

n  CPU registers

n  CPU scheduling information

n  Memory-management information

n  Accounting information

n  I/O status information

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Process Control Block (PCB)

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EPROCESS

… void *UniqueProcessId; …

NT Executive

Windows NT Process Descriptor

KPROCESS … uint32 KernelTime; uint32 UserTime; … Byte state;

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

2 3

4 5

7 Interrupt

8

9

6

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Context Switch

n  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.

n  Context-switch time is overhead; the system does no useful work while switching context.

n  Time dependent on hardware support.

How to reduce context-switch overheads?