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1.2.2. Advantages of Distributed Systems over Independent PCs. That is the theory. The practice is that to function at all, current distributed systems count on a number of specific servers being up. As a result, some of them have an availability more closely related to the Boolean and of the components than to the Boolean OR. In a widely-quoted remark, Leslie Lamport once defined a distributed system as “one on which I cannot get any work done because some machine I have never heard of has crashed.” While this remark was (presumably) made somewhat tongue-in-cheek, there is clearly room for improvement here.. When the workstations have private disks, these disks can be used in one of at least four ways:. To make this strategy work well,R must be large enough that the probability is high that enough workers will be found to handle the entire job. Otherwise the request will have to move up one level in the tree and start all over, wasting considerable time and computing power. On the other hand, ifR is too large, too many processors will be allocated, wasting computing capacity until word gets back to the top and they can be released.. When a process wants to start up a child process, it goes around and checks out who is currently offering the service that it needs. It then determines the set of processors whose services it can afford. From this set, it computes the best candidate, where “best” may mean cheapest, fastest, or best price/performance, depending on the application. It then generates a bid and sends the bid to its first choice. The bid may be higher or lower than the advertised price.. 4.6. REAL-TIME DISTRIBUTED SYSTEMS. Future distributed file systems will probably have to deal with changes in hardware technology, scalability, wide-area systems, mobile users, and fault tolerance, as well as the introduction of multimedia. Many exciting challenges await us.. 11. Consider a system that does client caching using the write-through algorithm. Individual blocks, rather than entire files, are cached. Suppose that a client is about to read an entire file sequentially, and some of the blocks are in the cache and others are not. What problem may occur, and what can be done about it?. A second important library procedure isgetload. It returns information about the CPU speed, current load, and amount of memory free at the moment. It is used by the run server to determine the best place to execute a new process. A third major library procedure isstun. A process’ parent can suspend it bystunningit. More commonly, the parent can give the process’ capability to a debugger, which can stun it and later restart it for interactive debugging purposes. Two kinds of stuns are supported: normal and emergency. They differ with respect to what happens if the process is blocked on one or more RPCs at the time it is stunned. With a normal stun, the process sends a message to the server it is currently waiting for, saying, in effect: “I have been stunned. Finish your work instantly and send me a reply.” If the server is also blocked, waiting for another server, the message is propagated further, all the way down the line to the end. The server at the end of the line is expected to reply immediately with a special error message. In this way, all the pending RPCs are terminated almost immediately in a clean way, with all of the servers finishing properly. The nesting structure is not violated, and no “long jumps” are needed.. This design was chosen to provide a greater degree of transparency than an open group structure would have. The idea behind it is that clients normally use RPC to talk to individual servers, so they should use RPC to talk to groups as well. The alternative— open groups and using RPC to talk to single servers but using group communication to talk to group servers — is much less transparent. (Using group communication for everything would eliminate the many advantages of RPC that we have discussed earlier.) Once it has been determined that clientsoutside a group will use RPC to talk to the group (actually, to talk to one process in the group), the need for open groups vanishes, so closed groups, which are easier to implement, are adequate.. Having described the main abstractions provided by the Chorus kernel, let us now briefly examine how the kernel is structured internally. The kernel consists of four pieces, as illustrated in Fig. 9-5. At the bottom is thesupervisor,which manages the raw hardware and catches traps, exceptions, interrupts, and other hardware details, and handles context switching. It is written partly in assembler and has to be redone when Chorus is ported to new hardware.. Messages are addressed to ports, each of which contains storage for a certain number of messages. If a message is sent to a port that is full, the sender is suspended until sufficient space is available. When a port is created, both a unique identifier and a local identifier are returned to the caller. The former can be sent to other processes so they can send messages to the port. The latter is used within the process to reference the port directly. Only threads in the process currently holding the port can read from the port (ports can migrate).. Amoeba, Mach, and Chorus all work on multiprocessors, but they differ on how they deal with threads on these machines. In Amoeba, all the threads of a process run on the same CPU, in pseudoparallel, timeshared by the kernel. In Mach processes have fine-grained control over which threads run on which CPUs (using the processor set concept). Consequently, the threads of the same process can run in parallel on different CPUs.. The threads package, along with the RPC package, is one of the fundamental building blocks of DCE. In this section we will discuss the DCE threads package, focusing on scheduling, synchronization, and the calls available.. Each attribute can be marked as PUBLIC, STANDARD, or SENSITIVE. It is possible to associate with each object access control lists specifying which users may read and which users may modify attributes in each of these three categories..