1.2.3. Disadvantages of Distributed Systems. 2. Let processes pick random addresses; locate them by broadcasting.. When the server begins executing, the call toinitialize outside the main loop [see Fig. 2-9(a)]exportsthe server interface. What this means is that the server sends a message to a program called abinder, to make its existence known. This process is referred to asregisteringthe server. To register, the server gives the binder its name, its version number, a unique identifier, typically 32 bits long, and ahandleused to locate it. The handle is system dependent, and might be an Ethernet address, an IP address, an X.500 address, a sparse process identifier, or something else. In addition, other information, for example, concerning authentication, might also be supplied. A server can also deregister with the binder when it is no longer prepared to offer service. The binder interface is shown in Fig. 2-23.. To start with, it can happen that the client cannot locate a suitable server. The server might be down, for example. Alternatively, suppose that the client is compiled using a particular version of the client stub, and the binary is not used for a considerable period of time. In the meantime, the server evolves and a new version of the interface is installed and new stubs are generated and put into use. When the client is finally run, the binder will be unable to match it up with a server and will report failure. While this mechanism is used to protect the client from accidentally trying to talk to a server that may not agree with it in terms of what parameters are required or what it is supposed to do, the problem remains of how this failure should be dealt with.. Efficiency is achieved by avoiding unnecessary transitions between user and kernel space. If a thread blocks on a local semaphore, for example, there is no reason to involve the kernel. The user-space runtime system can block the synchronizing thread and schedule a new one by itself.. 2. Centralized versus distributed algorithms.. The last issue in our list deals withlocation policy.Once the transfer policy has decided to get rid of a process, the location policy has to figure out where to send it. Clearly, the location policy cannot be local. It needs information about the load elsewhere to make an intelligent decision. This information can be disseminated in two ways, however. In one method, the senders start the information exchange. In another, it is the receivers that take the initiative.. Suppose elementA2fails. Each of the voters,V1, V2,andV3 gets two good (identical) inputs and one rogue input, and
each of them outputs the correct value to the second stage. In essence, the effect ofA2 failing is completed masked, so that the inputs toB1, B2, andB3 are exactly the same as they would have been had no fault occurred.. Using session semantics raises the question of what happens if two or more clients are simultaneously caching and modifying the same file. One solution is to say that as each file is closed in turn, its value is sent back to the server, so the final result depends on who closes last. A less pleasant, but slightly easier to implement alternative is to say that the final result is one of the candidates, but leave the choice of which one unspecified.. 3. To split the workload over multiple servers. As the system grows in size, having all the files on one server can become a performance bottleneck. By having files replicated on two or more servers, the least heavily loaded one can be used.. Next come the switched multiprocessors, such as the Stanford Dash machine and the M.I.T. Alewife machine. These also have hardware caching but use directories and other data structures to keep track of which CPUs or clusters have which cache blocks. Complex algorithms are used to maintain consistency, but since they are stored primarily in MMU microcode (with exceptions potentially handled in software), they count as mostly “hardware” implementations.. Not all 64 signature patterns are allowed. As a trivial example, 000000 is not permitted, because that would imply that the print statements ran before the assignment statements, violating Lamport’s requirement that statements are executed in program order. A more subtle example is 001001. The first two bits, 00, mean thatbandcwere both 0 whenP1 did its printing. This situation occurs only whenP1 executes both statements beforeP2 orP3 starts. The next two bits, 10, mean thatP2 must run afterP1 has started but beforeP3 has started. The last two bits, 01, mean thatP3, must complete beforeP1 starts, but we have already seen thatP1 must go first. Therefore, 001001 is not allowed. . [Картинка: any2fbimgloader155]. A common programming paradigm in Linda is thereplicated worker model.This model is based on the idea of atask bagfull of jobs to be done. the main process starts out by executing a loop containing. 7. Each IDL file contains a unique number (e.g., produced by theuuidgen program). What is the value of having this number?.