Книга: Distributed operating systems

9.7.1. Philosophy

9.7.1. Philosophy

Amoeba, Mach and Chorus have different histories and different philosophies. Amoeba was designed from scratch as a distributed system for use on a collection of CPUs connected by a LAN. Later, multiprocessors and WANs were added. Mach (actually RIG) started out as an operating system for a single processor, with multiprocessors and LANs being added afterward. Chorus began as a distributed operating systems research project quite far from the UNIX world, and went through three major revisions, getting closer and closer to UNIX each time. The consequences of these differing backgrounds are still visible in the systems.

Amoeba is based on the processor pool model. A user does not log into a particular machine, but into the system as a whole. The operating system decides where to run each command, based on the current load. It is normal for requests to use multiple processors; rarely will two consecutive commands run on the same processor. There is no concept of a home machine.

In contrast, Mach and Chorus (UNIX) were designed with the idea that a user definitely logs into a specific machine and runs all his programs there by default. There is no attempt to spread each user's work over as many machines as possible (although on a multiprocessor, the work will be spread automatically over all the multiprocessor's CPUs). While it is possible to run remotely, the philosophical difference is that each user has a home machine (e.g., a workstation) where most of his work is carried out. However, this distinction was later blurred when Mach was ported to the Intel Paragon, a machine consisting of a pool of processors.

Another philosophical difference relates to what a "microkernel" is. The Amoeba view follows the famous dictum expounded by the French aviator and writer Antoine de St. Exupery: Perfection is not achieved when there is nothing left to add, but when there is nothing left to take away. Whenever a proposal was made to add a new feature to the kernel, the deciding question was: Can we live without it? This philosophy led to a minimal kernel, with most of the code in user-level servers.

The Mach designers, in contrast, wanted to provide enough functionality in the kernel to handle the widest possible range of applications. In many areas, Amoeba contains one way to do something and Mach contains two or three, each more convenient or efficient under different circumstances. Consequently, the Mach kernel is much larger and has five times as many system calls (including calls to kernel threads) as Amoeba. Chorus occupies an intermediate position between Amoeba and Mach, but it still has more system calls than 4.2 BSD UNIX, which is hardly a microkernel. A comparison is given in Fig. 9-27.

Fig. 9-27. Number of system calls and calls to kernel threads in selected systems.

Another philosophical difference between Amoeba and Mach is that Amoeba has been optimized for the remote case (communication over the network) and Mach for the local case (communication via memory). Amoeba has extremely fast RPC over the network, while Mach's copy-on-write mechanism provides high-speed communication on a single node, for example. Chorus has primarily emphasized single CPU and multiprocessor systems, but since the communication management is done in the kernel (as in Amoeba) and not in a user process (as in Mach), it potentially can have good RPC performance, too.

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