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2.5. GROUP COMMUNICATION. In Fig. 3-22(g), gamma has changed the file and already committed. Again beta must abort. In Fig. 3-22(h), gamma is in the process of changing the file, although it has not committed yet. Still, beta is too late and must abort.. 14. In Fig. 3-17(d) three schedules are shown, two legal and one illegal. For the same transactions, give a complete list of all values thatx might have at the end, and state which are legal and which are illegal.. The one diskless and four diskful models we have discussed are summarized in Fig. 4-11. The progression from top to bottom in the figure is from complete dependence on the file servers to complete independence from them..  (c) GETPID (return the process id).. Let us consider the case of separate directory and file servers for the moment. In the normal case, the client sends a symbolic name to the directory server, which then returns the binary name that the file server understands. However, it is possible for a directory hierarchy to be partitioned among multiple servers, as illustrated in Fig. 5-7. Suppose, for example, that we have a system in which the current directory, on server 1, contains an entry,a, for another directory on server 2. Similarly, this directory contains an entry,b, for a directory on server 3. This third directory contains an entry for a filec, along with its binary name.. For example, if there are five servers and a client determines that three of them have version 8, it is impossible that the other two have version 9. After all, any successful update from version 8 to version 9 requires getting three servers to agree to it, not just two.. In the early days of distributed computing, everyone implicitly assumed that programs on machines with no physically shared memory (i.e., multicomputers) obviously ran in different address spaces. Given this mindset, communication was naturally viewed in terms of message passing between disjoint address spaces, as described above. In 1986, Li proposed a different scheme, now known under the namedistributed shared memory (DSM)(Li, 1986; and Li and Hudak, 1989). Briefly summarized, Li and Hudak proposed having a collection of workstations connected by a LAN share a single paged, virtual address space. In the simplest variant, each page is present on exactly one machine. A reference to alocal pages is done in hardware, at full memory speed. An attempt to reference a page on a different machine causes a hardware page fault, which traps to the operating system. The operating system then sends a message to the remote machine, which finds the

needed page and sends it to the requesting processor. The faulting instruction is then restarted and can now complete..   stack [top] := item;            # push item onto the stack. If the object is replicated, as in Fig. 6-42(c) and (d), a copy will always be available locally, but now it matters if the operation is a read or a write. Reads are just done locally, with no network traffic and thus no overhead.. A second implementation is to use one Mach thread per C thread, as shown in Fig. 8-5(b). These threads are scheduled preemptively. Furthermore, on a multiprocessor, they may actually run in parallel, on different CPUs. In fact, it is also possible to multiplexm user threads onn kernel threads, although the most common case ism = n.. Within a process, one or morethreadscan exist. Each thread is similar to a process in that it has its own stack, stack pointer, program counter, and registers. However, all threads in a process share the same address space and other process-wide resources. In principle, the threads of a process are independent of one another. On a multiprocessor, several threads may be running at the same time, on different CPUs. All three kinds of processes can have multiple threads.. Protection in DCE is closely tied to the cell structure. Each cell has onesecurity servicethat the local principals have to trust. The security service, of which the authentication server is part, maintains keys, passwords and other security-related information in a secure data base called theregistry. Since different cells can be owned by different companies, communicating securely from one cell to another requires a complex protocol, and can be done only if the two cells have set up a shared secret key in advance. For simplicity, we will restrict our subsequent discussion to the case of a single cell.. whereS is the server for whom the ticket is intended. The information within curly braces is encrypted using the server’s private key,KS. The fields encrypted include a temporary session key, the client’s identity, the time at which the ticket ceases to be valid, and a message identifier ornoncethat is used to relate requests and replies. When the server decrypts the ticket with its private key, it obtains the session key to use when talking to the client. In our descriptions below, we will omit all encrypted ticket fields except the session-key and client name.. [Картинка: any2fbimgloader261].

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