Книга: Distributed operating systems

9.1.1. History of Chorus

9.1.1. History of Chorus

Chorus started out at the French research institute INRIA in 1980, as a research project in distributed systems. It has since gone through four versions, numbered from 0 through 3. The idea behind Version 0 was to model distributed applications as a collection of actors, essentially structured processes, each of which alternated between performing an atomic transaction and executing a communication step. In effect, each actor was a macroscopic finite-state automaton. Each machine in the system ran the same kernel, which managed the actors, communication, files, and I/O devices. Version 0 was written in interpreted UCSD Pascal and ran on a collection of 8086s connected by a ring network. It was operational by mid-1982.

Version 1, which lasted from 1982 to 1984, focused on multiprocessor research. It was written for the French SM90 multiprocessor, which consisted of eight Motorola 68020 CPUs on a common bus. One of the CPUs ran UNIX; the other seven ran Chorus and used the UNIX CPU for system services and I/O. Multiple SM90s were connected by an Ethernet. The software was similar to Version 0, with the addition of structured messages and some support for fault tolerance. Version 1 was written in compiled, rather than interpreted, Pascal and was distributed to about a dozen universities and companies for experimental use.

Version 2 (1984-1986) was a major rewrite of the system, in C. It was designed to be system call compatible with UNIX at the source code level, meaning that it was possible to recompile existing UNIX programs on Chorus and have them run on it. The Version 2 kernel was completely redesigned, moving as much functionality as possible from it to user code, and turning the kernel into what is now regarded as a microkernel. The UNIX emulation was done by several processes, for handling process management, file management, and device management, respectively. Support was added for distributed applications, including remote execution and protocols for distributed naming and location.

Version 3 was started in 1987. This version marked the transition from a research system to a commercial product, as the Chorus designers left INRIA and formed a company, Chorus Systemes, to further develop and market Chorus. Numerous technical changes were made in Version 3, including further refinement of the microkernel and its relation to the rest of the system. The last vestiges of the actor model, with its atomic transactions, disappeared, and RPC (Remote Procedure Call) was introduced as the usual communication model. Kernel mode processes also appeared.

To make Chorus a viable commercial product, the ability to emulate UNIX was beefed up. Binary compatibility was added, so existing UNIX programs could be run without being recompiled. Part of the UNIX emulation, which had been in the microkernel, was moved to the emulation subsystem, which was simultaneously made more modular. Exception handling was changed to be able to handle UNIX signals correctly.

Its performance was improved. Also, the system was partially rewritten in C++. Furthermore, it was made more portable and implemented on a number of different architectures. Version 3 also borrowed many ideas from other distributed system microkernels, notably the interprocess communication system, virtual memory design, and external pagers from Mach, and the use of sparse capabilities for global naming and protection from Amoeba.

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