Lecture Outline

Aims and Objectives
Introduction to Amoeba
The Amoeba System Architecture
The Amoeba Microkernel
The Amoeba Servers: Introduction
Process Management
Memory Management
Communication
The Amoeba Servers

• Case Study of a Distributed Operating System

• Originated at a university in Holland, 1981

• Currently used in various EU countries

• Built from the ground up. UNIX emulation added later

• Goal was to build a transparent distributed operating system

• Resources, regardless of their location, are managed by the system, and the user is unaware of where processes are actually run

• Assumes that a large number of CPUsare available and that each CPU ha 10s of Mb of memory

• CPUs are organised into processor pools

  

• CPUs do not need to be of the same architecture (can mix SPARC, Motorola PowerPC, 680x0, Intel, Pentium, etc.)

• When a user types a command, system determines which CPU(s) to execute it on. CPUs can be timeshared.

• Terminals are X-terminals or PCs running X emulators

• The processor pool doesn't have to be composed of CPU boards enclosed in a cabinet, they can be on PCs, etc., in different rooms, countries,...

• Some servers (e.g., file servers) run on dedicated processors, because they need to be available all the time

• The Amoeba microkernel is used on all terminals (with an on-board processor), processors, and servers

• The microkernel

  manages processes and threads

  provides low-level memory management support

  supports interprocess communication (point-to-point and group)

  handles low-level I/O for the devices attached to the machine

• OS functionality not provided by the microkernel is performed by Amoeba servers

• To use a server, the client calls a stub procedure which marshalls parameters, sends the message, and blocks until the result comes back

Server Basics

• Amoeba uses capabilities

• Every OS data structure is an object, managed by a server

• To perform an operation on an object, a client performs an RPC with the appropriate server, specifying the object, the operation to be performed and any parameters needed.

• The operation is transparent (client does not know where server is, nor how the operation is performed)

• Capabilites

  To create an object the client performs an RPC with the server

  Server creates the object and returns a capability

  To use the object in the future, the client must present the correct capability

  

  The check field is used to protect the capability against forgery

• Object protection

  When an object is created, server generates random check field, which it stores both in the capability and in its own tables

  The rights bits in the capability are set to on

  The server sends the owner capability back to the client

  Creating a capability with restricted rights

  

  Client can send this new capability to another process

• All processes are objects protected by capabilities

• Processes are managed at 3 levels

  by process servers, part of the microkernel

  by library procedures which act as interfaces

  by the run server, which decides where to run the processes

• Process management uses process descriptors

  Contains:

  platform description

  process' owner's capability

  etc

• Designed with performance, simplicity and economics in mind

• Process occupies contiguous segments in memory

• All of a process is constantly in memory

• Process is never swapped out or paged

• Point-to-point (RPC) and Group

The File System

• Consists of the Bullet (File) Server, the Directory Server, and the Replication Server

The Bullet Server

• Designed to run on machines with large amounts of RAM and huge local disks

• Used for file storage

• Client process creates a file using the create call

• Bullet server returns a capability that can be used to read the file with

• Files are immutable, and file size is known at file creation time. Contiguous allocation policies used

The Directory Server

• Used for file naming

• Maps from ASCII names to capabilities

• Directories also protected by capabilities

• Directory server can be used to name ANY object, not just files and directories

The Replication Server

• Used for fault tolerence and performance

• Replication server creates copies of files, when it has time

Other Amoeba Servers

The Run Server

• When user types a command, two decisions have to be made

  On which architecture should the process be run?

  Which processor should be chosen?

• Run server manages the processor pools

  

• Uses processes process descriptor to identify appropriate target architecture

• Checks which of the available processors have sufficient memory to run the process

• Estimates which of the remaining processor has the most available compute power

The Boot Server

• Provides a degree of fault tolerance

• Ensures that servers are up and running

• If it discovers that a server has crashed, it attempts to restart it, otherwise selects another processor to provide the service

• Boot server can be replicated to guard against its own failure

Next Lecture...

Object-Oriented Operating Systems