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Network Operating System (1)

General structure of a network operating system.

1-19



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Network Operating System (2) rlogin machine

rcp machine1:file1 machine2:file2

A better approach

(in terms of convenience and sharing):

client server approach, shared global file

server



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Network Operating System (3)

Different clients may mount the servers in different places.

1.21



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Distributed Operating Systems

But no longer have shared memory

Provide message passing

Can try to provide distributed shared memory But tough to get acceptable performance



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Middleware and the Operating System

What is a distributed OS?

Presents users (and applications) with an

integrated computing platform that hides the

individual computers.

Has control over all of the nodes (computers) inthe network and allocates their resources to

tasks without user involvement.

In a distributed OS, the user doesn't know

(or care) where his programs are running. Examples:

Cluster computer systems

V system, SpritePrinciples of Operating Systems


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No distributed OS in general useUsers have much invested in their application

software

Users tend to prefer to have a degree ofautonomy

for their machines

Network OS provides autonomy

Middleware provides network-transparent access

resource

Combination of middleware and network OS



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Protection




Chapter 5: Operating System Support



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Operating SystemTasks: processing, storage and communication

Components: kernel, library, user-level services

Middlewareruns on a variety of OS-hardware combinations

remote invocations

Architecture

The relationship between OS and Middleware



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Encapsulation provide a set of operations that meet their clients needs

Protectionprotect resource from illegitimate access

Concurrent processing support clients access resource concurrently

Invocation mechanism: a means of accessing

an encapsulated resourceCommunication

Pass operation parameters and results between resource managers

Scheduling

Schedule the processing of the invoked operation

Functions that OS should provide for middleware



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Process manager

Handles the creation of and operations upon processes. Thread manager

Thread creation, synchronization and scheduling

Communication manager

Communication between threads attached to differentprocesses on the same computer

Memory manager Management of physical and virtual memory

Supervisor Dispatching of interrupts, system call traps and otherexceptions

control of memory management unit and hardware caches

processor and floating point unit register manipulations

Figure

The core OS components



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Positioning Middleware

Network OSs are not transparent.

Distributed OSs are not independent ofcomputers.

Middleware can help.



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Middleware

What is middleware?

What is its function?

Where is it located?

Why does it exist?



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Positioning Middleware

General structure of a distributed system as middleware.



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Figure 2.1

Software and hardware service

layers in distributed systems

Applications, serv ices

Computer and network hardware

Platf orm

Operating system

Middleware

Principles of OperatingSystems


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Middleware and Openness

In an open middleware-based distributed system, the protocols used byeach middleware layer should be the same, as well as the interfaces theyoffer to applications.

Why should they be open? Why should they not be open?

1.23



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Typical Middleware Services

Communication

Naming

Persistence

Distributed transactions

Security



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Middleware Models

Distributed files Examples?

Remote procedure call

Examples?

Distributed objects Examples?

Distributed documents

Examples? Others?

Message-oriented middleware (MOM)

Service oriented architecture (SOA)

Document-oriented



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Middleware and the Operating System

Middleware implements abstractions that support network-wide programming. Examples:

RPC and RMI (Sun RPC, Corba, Java RMI)

event distribution and filtering (Corba Event Notification, Elvin)

resource discovery for mobile and ubiquitous computing support for multimedia streaming

Traditional OS's (e.g. early Unix, Windows 3.0) simplify, protect and optimize the use of local resources

Network OS's (e.g. Mach, modern UNIX, Windows NT)

do the same but theyalso support a wide range of communicationstandards and enable remote processes to access (some) local resources

(e.g. files).



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DOS vs. NOS vs. Middleware

Discussion What is good/bad about DOS?

Transparency

Other issues have reduced success.

Problems are often socio-technological.

What is good/bad about NOS? Simple.

Decoupled, easy to add/remove.

Lack of transparency. What is good/bad about middleware?

Easy to make multiplatform.

Easy to start something new. But this can also be bad.



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Types of Distributed OSs

System Description Main Goal

DOS

Tightly-coupled operating system for multi-

processors and homogeneousmulticomputers

Hide and manage

hardwareresources

NOSLoosely-coupled operating system forheterogeneous multicomputers (LAN andWAN)

Offer localservices to remoteclients

MiddlewareAdditional layer atop of NOS implementinggeneral-purpose services

Provide

distributiontransparency



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Protection







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Maliciously contrived code Benign code

contains a bug

have unanticipated behavior

Example: readand writein File System

Illegal uservs. access right control

Access the file pointer variable directly

(setFilePointerRandomly) vs. type-safe language Typesafe language, e.g. Java or Modula-3

Non-type-safe language, e.g. C or C++

Illegitimate access



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The System Clock

system clock frequency

clock period(One full period is also called a clock cycle )

"Hertz" (Hz) meaning one cycle per second10 MHz : 100 nanoseconds

1GHz: 1 nanoseconds Principles of Operating Systems


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Protection







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Operation system concepts

(six edition)

Abraham Silberschats,

Peter Baer Galvin,

Greg Gagne Chapter 4 Processes

Chapter 5 Threads



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Single and Multithreaded Processes

Heavyweight process lightweight processPrinciples of Operating Systems


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Process

A program in execution Problem: sharing between related activities are awkward and

expensive

Nowadays, a process consists of an execution environment

together with one or more threads an analogy at page 215

Thread Abstraction of a single activity

Benefits Responsiveness

Resource sharing

Economy

Utilization of MP architectures

Process and thread



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the unit of resource management

Consist of An address space

Thread synchronization and communication resources such

as semaphores and communication interfaces (e.g. sockets) Higher-level resources such as open files and windows

Shared by threads within a process

Execution environment



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The number of regions is indefinite

Support a separate stack for each thread

access mapped file

Share memory between processes

Region can be shared

Libraries

Kernel

Shared data and communication

Copy-on-write

Address space continued



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Creating process by the operation

system

Fork, exec in UNIX

Process creation in distributed systemThe choice of a target host

The creation of an execution environment, an initial

thread

Creation of new process in distributed system



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Choice of process host

running new processes at their originators computer sharing processing load between a set of computers

Load sharing policy

Transferpolicy: situate a new process locally or remotely?

Location policy: which node should host the new process? Static policy without regard to the current state of the system

Adaptive policy applies heuristics to make their allocation decision

Migration policy: when&where should migrate the running

process? Load sharing system

Centralized

Hierarchical

Decentralized

Choice of process host



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Initializing the address space

Statically defined format

With respect to an existing execution environment,

e.g.fork

Copy-on-writescheme

Creation of a new execution environment



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Process

Thread activations

Activation stacks

(parameters, local variables)

Threads concept and implementation

'text' (program code)Heap (dynamic storage,

objects, global variables)

system-provided resources

(sockets, windows, open files)



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Client and server with threads

Client

Thread 2 makes

Thread 1

requests to server

generates

results

Server

N threads

Input-output

Requests

Receipt &queuing

*

The 'worker pool' architecture


Alt ti th di hit t


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Alternative server threading architectures

a. Thread-per-request b. Thread-per-connection c. Thread-per-object

remote

workers

I/O

objects

server

process

remote

per-connection threads

objects

server

process

remoteI/O

per-object threads

objects

server

process

(a) would be useful for UDP-based service, e.g. NTP

(b) is the most commonly used - matches the TCP connection model

(c) is used where the service is encapsulated as an object. E.g. could have

multiple shared whiteboards with one thread each. Each object has only

one thread, avoiding the need for thread synchronization within objects.



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Threads implementation


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Threads implementation

Threads can be implemented:

in the OS kernel (Win NT, Solaris, Mach) at user level (e.g. by a thread library: C threads,

pthreads), or in the language (Ada, Java).

+ lightweight - no system calls

+ modifiable scheduler

+ low cost enables more threads to be employed

- not pre-emptive

- can exploit multiple processors

- - page fault blocks all threads

hybrid approaches can gain some advantages of both user-level hints to kernel scheduler

hierarchic threads (Solaris 2)

event-based (SPIN, FastThreads)

*Principles of Operating Systems


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Introduction

The operating system layer

Protection







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Invocation costs

Different invocations

The factors that matter synchronous/asynchronous, domain transition, communication

across a network, thread scheduling and switching

Invocation over the network Delay: the total RPC call time experienced by a client

Latency: the fixed overhead of an RPC, measured by null

RPC

Throughput: the rate of data transfer between computers in asingle RPC

An example

Threshold: one extra packet to be sent, might be an extra

acknowledge packet is needed

Invocation performance



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The performance of RPC and RMI mechanisms is critical for effective

distributed systems.

Typical times for 'null procedure call':

Local procedure call < 1 microseconds

Remote procedure call ~ 10 milliseconds

'network time' (involving about 100 bytes transferred, at 100 megabits/sec.)accounts for only .01 millisecond; the remaining delays must be in OS and

middleware - latency, not communication time.

Factors affecting RPC/RMI performance

marshalling/unmarshalling + operation despatch at the server

data copying:- application -> kernel space -> communication buffers

thread scheduling and context switching:- including kernel entry

protocol processing:- for each protocol layer

network access delays:- connection setup, network latency

10,000 times slower!

Support for communication and invocation



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Memory sharing rapid communication between processes in the

same computer

Choice of protocol

TCP/UDP E.g. Persistent connections: several invocations duringone

OSs buffercollect several small messages andsend them together

Invocation within a computer Most cross-address-space invocation take place

within a computer

LRPC (lightweight RPC)

Improve the performance of RPC



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Performance characteristics of the Internet

High latencies, low bandwidths and high server loads

Network disconnection and reconnection.

outweigh any benefits that the OS can provide

Asynchronous operation Concurrent invocations

E.g., the browser fetches multiple images in a home page by

concurrent GETrequests

Asynchronous invocation: non-blocking call

E.g., CORBA oneway invocation: maybe semantics, or

collect result by a separate call

Asynchronous operation



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Persistent asynchronous invocations

Designed fordisconnected operation

Try indefinitely to perform the invocation, until it is

known to have succeeded or failed, or until the

application cancels the invocation QRPC (Queued RPC)

Client queues outgoing invocation requests in a stable log

Server queues invocation results

The issues to programmers How user can continue while the results of

invocations are still not known?

Asynchronous operation continued



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Introduction

The operating system layer

Protection







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S l


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System layers

Applica tions, serv ic es

Computer &

Platf orm

Middleware

OS: kernel,libraries &servers

network hardware

OS1

Computer &network hardware

Node 1 Node 2

Processes, threads,communication, . ..

OS2Processes, threads,communication, ...

Figure 2.1

Software and hardware service layers in distributed systems

Applications, serv ices

Computer and network hardware

Platf orm

Operating system

Middleware



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P dd


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Process address space

Stack

Text

Heap

Auxiliaryregions

0

2

N

Regions can be shared

kernel code

libraries

shared data & communication

copy-on-write

Files can be mapped

Mach, some versions of UNIX

UNIX fork() is expensive

must copy process's address space



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State associated with execution environments and threads


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State associated with execution environments and threads

Execution environment Thread

Address space tables Saved processor registers

Communication interfaces, open files Priority and execution state (such as

BLOCKED)

Semaphores, other synchronization

objects

Software interrupt handling information

List of thread identifiers Execution environment identifier

Pages of address space resident in memory; hardware cache entries


Java Thread constructor and management methods


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Java Threadconstructor and management methods

Thread(ThreadGroup group, Runnable target, String name)

Creates a new thread in the SUSPENDED state, which will belong togroup and be

identified as name; the thread will execute the run() method oftarget.

setPriority(int newPriority), getPriority()

Set and return the threads priority.

run()

A thread executes the run() method of its target object, if it has one, and otherwise itsown run() method (ThreadimplementsRunnable).

start()

Change the state of the thread from SUSPENDED toRUNNABLE.

sleep(int millisecs)

Cause the thread to enter the SUSPENDED state for the specified time.

yield()Enter theREADYstate and invoke the scheduler.

destroy()

Destroy the thread.


I ti b t dd


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Invocations between address spaces

Control transfer via

trap instruc tion

User Kernel

Thread

User 1 User 2

Control transfer via

privileged instructions

Thread 1 Thread 2

Protection domain

boundary

(a) Sy stem c all

(b) R PC/RMI (within one computer)

Kernel

(c) R PC/RMI (between computers)

User 1 User 2

Thread 1 Network Thread 2

Kernel 2Kernel 1


Delay of an RPC operation when returned data size varies


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Delay of an RPC operation when returned data size varies

1000 2000

RPC delay

Requested dat

size (bytes)

Packet

size

0


A li ht i ht t d ll


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A lightweight remote procedure call

1. Copy args

2. Trap to Kernel

4. Execute procedureand copy results

Client

User stub

Serv er

Kernel

stub

3. Upcall 5. Return (trap)

AA stack


Times for serialized and concurrent invocations


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Times for serialized and concurrent invocations

Client Serv er

execute request

Send

Receiv eunmarshal

marshal

Receiv eunmarshal

process results

marshalSend

process args

marshal

Send

process args

transmission

Receiv eunmarshal

process results

execute request

Send

Receiv eunmarshal

marshal

marshal

Send

process args

marshalSend

process args

execute reques

Send

Receiv eunmarshal

marshal

execute reques

Send

Receiv eunmarshal

marshalReceiv e

unmarshalprocess results

Receiv eunmarshal

process results

time

Client Serv er

Serialised inv ocations Concurrent inv ocations


Monolithic kernel and Microkernel


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Monolithic kernel and Microkernel

Monolithic Kernel Microkernel

Serv er: Dy namically loaded server program:Kernel code and data:

.......

.......

Key:

S4

S1.......

S1 S2 S3S2 S3 S4

Middleware

Language

supportsubsystem

Language

supportsubsystem

OS emulation

subsystem....

Microkernel

Hardware

The microkernel supports middleware via subsystems

network organization

Documents