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As an aside, if a process blocks for I/O on a multiprocessor, the operating system has the choice of suspending it or just letting it do busy waiting. If most I/O is completed in less time than it takes to do a process switch, busy waiting is preferable. Some systems let the process keep its processor for a few milliseconds, in the hope that the I/O will complete soon, but if that does not occur before the timer runs out, a process switch is made (Karlin et al., 1991). If most critical regions are short, this approach can avoid many expensive process switches.. Transparency can be achieved at two different levels. Easiest to do is to hide the distribution from the users. For example, when a UNIX user typesmake to recompile a large number of files in a directory, he need not be told that all the compilations are proceeding in parallel on different machines and are using a variety of file servers to do it. To him, the only thing that is unusual is that the performance of the system is halfway decent for a change. In terms of commands issued from the terminal and results displayed on the terminal, the distributed system can be made to look just like a single-processor system.. Each of these categories poses different problems and requires different solutions.Client Cannot Locate the Server.  (c)  Remote banking.. Just think, for a moment, about the implications of the lack of global time on the UNIXmake program, as a single example. Normally, in UNIX, large programs are split up into multiple source files, so that a change to one source file only requires one file to be recompiled, not all the files. If a program consists of 100 files, not having to recompile everything because one file has been changed greatly increases the speed at which programmers can work.. The computer model is similar. One process announces that it wants to begin a transaction with one or more other processes. They can negotiate various options, create and delete objects, and perform operations for a while. Then the initiator announces that it wants all the others to commit themselves to the work done so far. If all of them agree, the results are made permanent. If one or more processes refuse (or crash before agreement), the situation reverts to exactly the state it had before the transaction began, with all side effects on objects, files, data bases, and so on, magically wiped out. This all-or-nothing property eases the programmer’s job.. A second reason that diskless workstations are popular is their ease of maintenance. When a new release of some program, say a compiler, comes out, the system administrators can easily install it on a small number of file servers in the machine room. Installing it on dozens or hundreds of machines all over a building or campus is another matter entirely. Backup and hardware maintenance is also simpler with one centrally located 5-gigabyte disk than with fifty 100-megabyte disks scattered over the building.. Distributed real-time systems can often be structured as illustrated in Fig. 4-26. Here we see a collection of computers connected by a network. Some of these are connected to external devices that produce or accept data or expect to be controlled in real time. The computers may be tiny microcontrollers built into the devices, or stand-alone machines. In both cases they usually have sensors for receiving signals from the devices and/or actuators for sending signals to them. The sensors and actuators may be digital or analog.. [Картинка: any2fbimgloader146]. [Картинка: any2fbimgloader149]. The idea of having memory be wrong is nothing new. Many compilers cheat too. For example, consider the program fragment of Fig. 6-21, with all the variables initialized to appropriate values. An optimizing compiler may decide to computea andb in registers and keep the values there for a while, not updating their memory locations. Only when the function / is called does the compiler have to put the current values ofa andb back in memory, because / might try to access them.. Two features of Orca important for distributed programming are shared data-objects (or justobjects) and theforkstatement. An object is an abstract data type, somewhat analogous to a package in Ada®. It encapsulates internal data structures and user-written procedures, calledoperations(ormethods) for operating on the internal data structures. Objects are passive, that is, they do not contain threads to which messages can be sent. Instead, processes access an object’s internal data by invoking its operations. Objects do not inherit properties from other objects, so Orca is considered an object-based language rather than an object-oriented language.. Mach scheduling has been heavily influenced by its goal of running on multiprocessors. Since a single-processor system is effectively a special case of a multiprocessor (with only one CPU), our discussion will focus on scheduling in multiprocessor systems. For more information, see (Black, 1990).. Mach messages can be either simple or complex. Simple messages are just bits and are not processed in any special way by the kernel. Complex messages may contain capabilities. They may also pass out-of-line data using copy-on-write, something Amoeba does not have. On the other hand, this facility is of little value in a distributed system because the out-of-line data must be fetched by the network message server, combined with the message header and in-line data, and sent over the network. This optimization is for the local case and wins nothing when the sender and receiver are on different machines.. Fig. 10-14. Client-to-server binding in DCE..