Книга: Distributed operating systems

1.4.2. True Distributed Systems

1.4.2. True Distributed Systems

Network operating systems are loosely-coupled software on loosely-coupled hardware. Other than the shared file system, it is quite apparent to the users that such a system consists of numerous computers. Each can run its own operating system and do whatever its owner wants. There is essentially no coordination at all, except for the rule that client-server traffic must obey the system's protocols.

The next evolutionary step beyond this is tightly-coupled software on the same loosely-coupled (i.e., multicomputer) hardware. The goal of such a system is to create the illusion in the minds of the users that the entire network of computers is a single timesharing system, rather than a collection of distinct machines. Some authors refer to this property as the single-system image. Others put it slightly differently, saying that a distributed system is one that runs on a collection of networked machines but acts like a virtual uniprocessor. No matter how it is expressed, the essential idea is that the users should not have to be aware of the existence of multiple CPUs in the system. No current system fulfills this requirement entirely, but a number of candidates are on the horizon. These will be discussed later in the book.

What are some characteristics of a distributed system? To start with, there must be a single, global interprocess communication mechanism so that any process can talk to any other process. It will not do to have different mechanisms on different machines or different mechanisms for local communication and remote communication. There must also be a global protection scheme. Mixing access control lists, the UNIX® protection bits, and capabilities will not give a single system image.

Process management must also be the same everywhere. How processes are created, destroyed, started, and stopped must not vary from machine to machine. In short, the idea behind network operating systems, namely that any machine can do whatever it wants to as long as it obeys the standard protocols when engaging in client-server communication, is not enough. Not only must there be a single set of system calls available on all machines, but these calls must be designed so that they make sense in a distributed environment.

The file system must look the same everywhere, too. Having file names restricted to 11 characters in some locations and being unrestricted in others is undesirable. Also, every file should be visible at every location, subject to protection and security constraints, of course.

As a logical consequence of having the same system call interface everywhere, it is normal that identical kernels run on all the CPUs in the system. Doing so makes it easier to coordinate activities that must be global. For example, when a process has to be started up, all the kernels have to cooperate in finding the best place to execute it. In addition, a global file system is needed.

Nevertheless, each kernel can have considerable control over its own local resources. For example, since there is no shared memory, it is logical to allow each kernel to manage its own memory. For example, if swapping or paging is used, the kernel on each CPU is the logical place to determine what to swap or page. There is no reason to centralize this authority. Similarly, if multiple processes are running on some CPU, it makes sense to do the scheduling right there, too.

A considerable body of knowledge is now available about designing and implementing distributed operating systems. Rather than going into these issues here, we will first finish off our survey of the different combinations of hardware and software, and come back to them in Sec. 1.5.

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