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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.. Fig. 2-7. The client-server model. Although all message passing is actually done by the kernels, this simplified form of drawing will be used when there is no ambiguity.. In our example, the file server has been assigned a numerical address (243), but we have not really specified what this means. In particular, does it refer to a specific machine, or to a specific process? If it refers to a specific machine, the sending kernel can extract it from the message structure and use it as the hardware address for sending the packet to the server. All the sending kernel has to do then is build a frame using the 243 as the data link address and put the frame out on the LAN. The server’s interface board will see the frame, recognize 243 as its own address, and accept it.. We could change the protocol to have acknowledgements themselves acknowledged, but this adds extra complexity and overhead for very little potential gain. In practice, the server can start a timer when sending the reply, and discard the reply when either the acknowledgement arrives or the timer expires. Also, a new request from the same client can be interpreted as a sign that the reply arrived, otherwise the client would not be issuing the next request.Critical Path. [Картинка: any2fbimgloader72]. The goals of the scheduler activation work are to mimic the functionality of kernel threads, but with the better performance and greater flexibility usually associated with threads packages implemented in user space. In particular, user threads should not have to be make special nonblocking system calls or check in advance if it is safe to make certain system calls. Nevertheless, when a thread blocks on a system call or on a page fault, it should be possible to run other threads within the same process, if any are ready.. Another technique to speed up RPCs is based on the observation that when a server thread blocks waiting for a new request, it really does not have any important context information. For example, it rarely has any local variables, and there is typically nothing important in its registers. Therefore, when a thread has finished carrying out a request, it simply vanishes and its stack and context information are discarded.. The biggest argument for centralizing the computing power in a processor pool comes from queueing theory. A queueing system is a situation in which users generate random requests for work from a server. When the server is busy, the users queue for service and are processed in turn. Common examples of queueing systems are bakeries, airport check-in counters, supermarket check-out counters, and numerous others. The bare basics are depicted in Fig. 4-14.. When a process references a word that is absent, it causes a page fault. An obvious choice is to bring in the entire page that is needed. Furthermore, integrating DSM with virtual memory makes the total design simpler, since the same unit, the page, is used for both. On a page fault, the missing page is just brought in from another machine instead of from the disk, so much of the page fault handling code is the same as in the traditional case.. Object implementationstack;   # variable indicating the top. The other form of communication is group communication. It allows a message to be sent from one source to multiple destinations. Software protocols provide reliable, fault-tolerant group communication to user processes in the presence of lost messages and other errors.. Chorus provides UNIX processes with many extensions to make distributed programming easier. Most of these are just standard Chorus properties that are made visible. For example, UNIX processes can create and destroy new threads using the Chorus threads package. These threads run in quasiparallel (on a multiprocessor, actually in parallel).. 1. Capabilities in Chorus use epoch numbers in their UIs. Why?. Fig. 10-15. Displaying time in DTS.. Fig. 10-31. Parts of DFS. (a) File client machine. (b) File server machine..