Experience with the Amoeba Distributed Operating System

Overview

• Intended to be a modern operating system (use the best hardware)
• Microkernel architecture
• Most of the traditional "kernel services" implemented as servers
• Extensive use of RPC to facilitate the distribution of services
• Transparency for the user (good or bad thing?)
• Creates the impression for user and processes that the collection of computers is like a big computer
• Trades resources for speed (see bullet file server, no virtual memory)
• Relies on the fact that at the time of design the bottle neck was the CPU and I/O not the network so an RPC was comparable with a system call

System architecture

• Four components: workstations (one per user, disk-less), pool processors (group of CPUs), specialized servers(file, directory, boot servers), gateways (for WAN interface)
• All the machines run the same kernel (multithreaded processes, communication, I/O)
• Support for multiple processor types (binary program files specify processor type they can execute on)
• User level (cooperative) multithreading
• Anything is an object (files, memory segments, processes, etc.). Every object has attached at least a capability (contains serverid, objectid, rights bitmaps and checksum). Capabilities are manipulated in user level so encryption is necessary (usage of one way functions)
• Powerful and simple RPC implementation. Has only three primitives: get_request, put_request and do_operation. Stubs can be generated with AIL language
• The system is much more transparent compared with the NFS mounting (IS IT?)
• Communication over WAN is transparent for user and processes. Clever use of the gateways that intercept and transfer a message to the other side.

Servers

• Memory and processor server: process creation, task distribution
• Bullet server: the file server especially designed to be fast. Contiguous file storage, caching at the level of file. Immutable files accessed through capabilities (no naming service)
• Directory server: offers naming for the file server (but not only).  Offers limited Access Control List like mechanisms (multiple capabilities for an object).

Achievements

• The performance is great. Very fast RPC (impressive results).
• The idea to base anything on RPC seems good especially if RPC is fast
• Very fast file system. Separating naming and storage and using immutability for files give the boost in performance.
• Reasonable good UNIX emulation

Drawbacks

• No virtual memory (serious limitation for some classes of applications)
• No preemptive tasks
• No communication except RPC (not clear how to emulate streams, pipes, etc. with RPC)
• Possible problems for large system (some of the services do not scale well)
• Garbage collection is a problem
• Hard to interface with something else that's not Amoeba