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A third reason to connect a group of isolated computers into a distributed system is to achieve enhanced person-to-person communication. For many people, electronic mail has numerous attractions over paper mail, telephone, and FAX. It is much faster than paper mail, does not require both parties to be available at the same time as does the telephone, and unlike FAX, produces documents that can be edited, rearranged, stored in the computer, and manipulated with text processing programs.. [Картинка: any2fbimgloader23]. On the other hand, if the request requires extensive computation on the part of the server, it would be a pity to discard the answer before the server is sure that the client has received the reply. For this reason, an acknowledgement from the client’s kernel to the server’s kernel is sometimes used. Until this packet is received, the server’ssend does not complete and the server remains blocked (assuming blocking primitives are used). In any event, if the reply is lost and the request is retransmitted, the server’s kernel can see that the request is an old one and just send the reply again without waking up the server. Thus in some systems the reply is acknowledged and in others it is not [see Fig. 2-13(b)].. One obvious, but unfortunately incorrect, approach is to invert the bytes of each word after they are received, leading to Fig. 2-20(c). Now the integer is 5 and the string is “LLIJ”. The problem here is that integers are reversed by the different byte ordering, but strings are not. Without additional information about what is a string and what is an integer, there is no way to repair the damage.. Fig. 2-22. A specification of the stateless server of Fig. 2-9.. 17. In Fig. 3-20, at what instant is the point-of-no-return reached? That is, when is the atomic commit actually performed?. 4.2.2. Using Idle Workstations. The motivation for the processor pool idea comes from taking the diskless workstation idea a step further. If the file system can be centralized in a small number of file servers to gain economies of scale, it should be possible to do the same thing for compute servers. By putting all the CPUs in a big rack in the machine room, power supply and other packaging costs can be reduced, giving more computing power for a given amount of money. Furthermore, it permits the use of cheaper X terminals (or even ordinary ASCII terminals), and decouples the number of users from the number of workstations. The model also allows for easy incremental growth. If the computing load increases by 10 percent, you can just buy 10 percent more processors and put them in the pool.. [Картинка: any2fbimgloader114]. Many clients are configured to mount certain remote directories without manual intervention. Typically, these clients contain a file called /etc/rc, which is a shell script containing the remote mount commands. This shell script is executed automatically when the client is booted.. mean that a write tox with the valuea and a read fromy returningb have been done, respectively. The initial value of all variables in these diagrams throughout this chapter is assumed to be 0. As an example, in Fig. 6-12(a)P1does a write to locationx,storing the value 1. Later,P2 readsxand sees the 1. This behavior is correct for a strictly consistent memory. . Fig. 6-19. Statement execution as seen by three processes. The statements marked with asterisks are the ones that actually generate output.. Much of the initial work on Mach concentrated on single– and multiprocessor systems. At the time Mach was designed, few systems had support for multiprocessors. Even now, few multiprocessor systems other than Mach are machine independent.. Various implementations of C threads are available on Mach. The original one did everything in user space inside a single process. This approach timeshared all the C threads over one kernel thread, as shown in Fig. 8-5(a). This approach can also be used on UNIX or any other system that provides no kernel support. The threads were run as coroutines, which means that they were scheduled nonpreemptively. A thread could keep the CPU as long as it wanted or was able to. For the producer-consumer problem, the producer would eventually fill the buffer and then block, giving the consumer a chance to run. For other applications, however, threads had to callyield from time to time to give other threads a chance.. The COOL generic runtime uses clusters and context spaces to manage objects. Objects are persistent and exist on disk when no process has them mapped in. Operations are provided to the language runtime system to create and delete objects, map them into and out of address spaces, and invoke their methods. When an object invokes a method of another object that is located in its own cluster, the invocation is done by a local procedure call. When the invoked object is in a different cluster, the generic runtime system is used to ask the COOL base to arrange for the remote invocation. Because the overhead of intracluster invocations is so much smaller than that of intercluster invocations, putting objects that invoke each other a lot together in one cluster greatly reduces system overhead. In COOL-1, all invocations were effectively intercluster operations, which proved to be expensive..