Книга: Distributed operating systems


Although the hardware is important, the software is even more important. The image that a system presents to its users, and how they think about the system, is largely determined by the operating system software, not the hardware. In this section we will introduce the various types of operating systems for the multiprocessors and multicomputers we have just studied, and discuss which kind of software goes with which kind of hardware.

Operating systems cannot be put into nice, neat pigeonholes like hardware. By nature software is vague and amorphous. Still, it is more-or-less possible to distinguish two kinds of operating systems for multiple CPU systems: loosely coupled and tightly coupled. As we shall see, loosely and tightly-coupled software is roughly analogous to loosely and tightly-coupled hardware.

Loosely-coupled software allows machines and users of a distributed system to be fundamentally independent of one another, but still to interact to a limited degree where that is necessary. Consider a group of personal computers, each of which has its own CPU, its own memory, its own hard disk, and its own operating system, but which share some resources, such as laser printers and data bases, over a LAN. This system is loosely coupled, since the individual machines are clearly distinguishable, each with its own job to do. If the network should go down for some reason, the individual machines can still continue to run to a considerable degree, although some functionality may be lost (e.g., the ability to print files).

To show how difficult it is to make definitions in this area, now consider the same system as above, but without the network. To print a file, the user writes the file on a floppy disk, carries it to the machine with the printer, reads it in, and then prints it. Is this still a distributed system, only now even more loosely coupled? It's hard to say. From a fundamental point of view, there is not really any theoretical difference between communicating over a LAN and communicating by carrying floppy disks around. At most one can say that the delay and data rate are worse in the second example.

At the other extreme we might find a multiprocessor dedicated to running a single chess program in parallel. Each CPU is assigned a board to evaluate, and it spends its time examining that board and all the boards that can be generated from it. When the evaluation is finished, the CPU reports back the results and is given a new board to work on. The software for this system, both the application program and the operating system required to support it, is clearly much more tightly coupled than in our previous example.

We have now seen four kinds of distributed hardware and two kinds of distributed software. In theory, there should be eight combinations of hardware and software. In fact, only four are worth distinguishing, because to the user, the interconnection technology is not visible. For most purposes, a multiprocessor is a multiprocessor, whether it uses a bus with snoopy caches or uses an omega network. In the following sections we will look at some of the most common combinations of hardware and software.

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