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Once all the terminals are in place, the possibility of also using them for electronic mail (especially in conjunction with printers) is clearly present. Since postal services lose a huge amount of money in every country in the world, and telephone services are enormously profitable, there are great incentives to having electronic mail replace paper mail.. 2.1.6. The Presentation Layer. }. Fig. 2-30. (a) Point-to-point communication is from one sender to one receiver. (b) One-to-many communication is from one sender to multiple receivers.. [Картинка: any2fbimgloader48]. [Картинка: any2fbimgloader57]. If a process that was previously down comes back up, it holds an election. If it happens to be the highest-numbered process currently running, it will win the election and take over the coordinator’s job. Thus the biggest guy in town always wins, hence the name “bully algorithm.”. Deadlocks in distributed systems are similar to deadlocks in single-processor systems, only worse. They are harder to avoid, prevent, or even detect, and harder to cure when tracked down because all the relevant information is scattered over many machines. In some systems, such as distributed data base systems, they can be extremely serious, so it is important to understand how they differ from ordinary deadlocks and what can be done about them.. Fig. 3-24. The Chandy-Misra-Haas distributed deadlock detection algorithm.. For the sake of generality, we will define agreement in a slightly different way here. Each general is assumed to know how many troops he has. The goal of the problem is for the generals to exchange troop strengths, so that at the end of the algorithm, each general has a vector of lengthn corresponding to all the armies. If generali is loyal, then elementi is his troop strength; otherwise, it is undefined.. A sequentially consistent memory can be implemented on a DSM or multiprocessor system that replicates writable pages by ensuring that no memory operation is started until all the previous ones have been completed. In a system with an efficient, totally-ordered reliable broadcast mechanism, for example, all shared variables could be grouped together on one or more pages, and operations to the shared pages could be broadcast. The exact order in which the operations are interleaved does not matter as long as all processes agree on the order of all operations on the shared memory. . The method of generating restricted capabilities is illustrated in Fig. 7-4. In this example, the owner has turned off all the rights except one. For example, the restricted capability might allow the object to be read, but nothing else. The meaning of theRights field is different for each object type since the legal operations themselves also vary from object type to object type.. The main practical value of the first approach is that because there is no true parallelism, successive runs give reproducible results, allowing easier debugging. The second approach has the advantage of simplicity and was used for a long time. The third one is not normally used. The fourth one, although the most complicated, gives the greatest flexibility and is the one normally used at present.. Now suppose that one of the readers tries to write the page. The DSM server sends a message to the kernel or kernels that have the page asking for it back. The page itself need not be transferred, because the DSM server has a valid copy itself. All that is needed is an acknowledgement that the page is no longer in use. When all the kernels have released the page, the writer is given a copy along with exclusive permission to use it (for writing).. In Amoeba and Chorus, synchronization between threads is done by mutexes and semaphores. In Mach it is done by mutexes and condition variables. Amoeba and Mach support some form of glocal variables. Chorus uses software registers to define each thread’s private context..