Distributed Information Systems Lecture 8 - Operating Systems, Lecture notes of Database Management Systems (DBMS)

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Operating Systems

Dr Simon Blake

Significance of this lecture

• This lecture focuses on a node and their

central control mechanisms

• How is software linked to physical

hardware?

• What controls exist for our physical

resources?

• This lesson is concerned with important

operating system concepts that impact ondistributed system design and behaviour.

The DIS model

-^

Resource sharing is an important aspect of a DIS

-^

What is involved in sharing?^ –

Clients invoke operations on resources on remote nodes or other localprocesses

-^

This process is simplified through the use of middleware

-^

In a DIS client applications/services send request to the middleware

-^

The middleware then sends its requests to the host operating system(OS)

-^

The OS then utilizes computer and network hardware to send requests

-^

So “What is the role of the OS?”^ –

The OS provide an abstraction of the hardware (network and computer)

-^

i.e. processors, memory, communications, storage, etc.

-^

Middleware talks to the OS rather than the hardware directly

Layers of a computer

Application, services

Middleware

OS

Computer / network hardware

OS layers

-^

Required functionality of an OS^ –

Encapsulation:

-^

With a

useful

set of operations to clients

-^

Unnecessary details should be transparent (memory management)

-^

Protection:

-^

protection to resources from incorrect and illegitimate access

-^

Concurrent processing:

-^

transparent concurrent process management

-^

With respect to remote invocations an OS also needs the followingfunctionality:^ –

Communication:

-^

to remote processes or at least other processes within the computer

-^

Scheduling

-^

Invoked operations need to be schedules to be processed.

Encapsulation

requests

requests

requests

Operating system

Accessvia theexportedmethods

Protection

Memory management Other OS functions

Encapsulationboundary

OS layers

-^

Process manager:^ –

Manages the creation and operations of processes

-^

Thread manager:^ –

Creates, synchronizes and schedules threads

-^

Communication manager:^ –

Communication between threads on the same host

-^

Memory manager:^ –

Physical and virtual memory

-^

Supervisor:^ –

The base layer - the hardware abstraction layer

-^

Interrupts, system traps, register manipulation, control of the memorymanagement, processor control

-^

Micro kernel^ – Implement as much as possible outside the kernel– Execute less code in Kernel Mode– Leads to more context switches– Requires inter process messaging

-^

Monolithic kernel^ – Nearly all functions exist within the kernel– Executes more code executed in kernel mode– Kernels can become very large– Faster execution– Has high function and code dependence

Micro/Monolithic kernels

Safeguards and protection

-^

Resources require protection from illegitimateaccess^ – illegitimate access is access without privileges

• An operation is performed that the client does not have

access rights to perform

• illegitimate access is access in unplanned way
  • An operation is performed that is not permitted at all -^

Protection is mainly the responsibility of the OSkernel

Safeguards and protection

-^

Features of the OS Kernel:^ –

It is always running,

-^

Its code has complete access to the physical resources of the computer

-^

Processors, memory, etc.

-^

The kernel allocates resources to processes^ –

For example, memory is allocated to user processes and they can onlyaccess memory inside its allocated region of memory

-^

Kernel protection^ –

User

processes run their own code in their allocated resource space

-^

However

user

processes can also run kernel code

-^

They do this through calling kernel supplied handlers

-^

These handlers control the access safeguarding against illicit activities

-^

Using interrupts or machine-level traps

Processes and threads

•^

Creating a process^ – Creating the process and its execution environment– More complex in a DIS

• Chose a target host• Maybe determined by policy
  • i.e. Always at originator– Could be based on load balancing (see pp217-218)

Create the execution environment on the selectedhost^ • Two approaches:

• Share the address space of the existing execution environment– Create a new address space

Processes and threads

• Threads (Threads of execution)

– Exist and run within a process execution

environment

– Cannot access threads in other processes– Can be dynamically created and destroyed by

the process

– Allow process to perform concurrent

operations

Server threading

-^

Thread per request^ –

A new thread is generated for each request

-^

They are destroyed when the request completes

-^

Increases throughput but introduces creation and destruction overheads

-^

This is managed via an I/O thread

Remoteobjects

Workerthreads

I/Othread

Server threading

-^

Thread per connection^ –

Creates a new thread for each connection that is made to the server

-^

Connection could make many requests through its single thread

Remoteobjects

ThreadPerconnection