The result of these technologies is that it is now not only feasible, but easy, to put together computing systems composed of large numbers of CPUs connected by a high-speed network. They are usually calleddistributed systems,in contrast to the previouscentralized systems(orsingle-processor systems) consisting of a single CPU, its memory, peripherals, and some terminals.. m1.offset = position; /* current position in the file */. If there is only one process running on the destination machine, the kernel will know what to do with the incoming message— give it to the one and only process running there. However, what happens if there are several processes running on the destination machine? Which one gets the message? The kernel has no way of knowing. Consequently, a scheme that uses network addresses to identify processes means that only one process can run on each machine. While this limitation is not fatal, it is sometimes a serious restriction.. int create(in char[MAX_PATH], in int mode);. Now we come to the broadcast primitives used in ISIS. Three of them have been defined: ABCAST, CBCAST, and GBCAST, all with different semantics. ABCAST provides loosely synchronous communication and is used for transmitting data to the members of a group. CBCAST provides virtually synchronous communication and is also used for sending data. GBCAST is somewhat like ABCAST, except that it is used for managing group membership rather than for sending ordinary data.. 3.1.3. Clock Synchronization Algorithms. Let us start with an algorithm that is well suited to systems in which one machine has a WWV receiver and the goal is to have all the other machines stay synchronized with it. Let us call the machine with the WWV receiver atimeserver.Our algorithm is based on the work of Cristian (1989) and prior work. Periodically, certainly no more than every?/2? seconds, each machine sends a message to the time server asking it for the current time. That machine responds as fast as it can with a message containing its current time,CUTC, as shown in Fig. 3-6.. If a process is handed the token by its neighbor and is not interested in entering a critical region, it just passes it along. As a consequence, when no processes want to enter any critical regions, the token just circulates at high speed around the ring.. The advantages of the workstation model are manifold and clear. The model is certainly easy to understand. Users have a fixed amount of dedicated computing power, and thus guaranteed response time. Sophisticated graphics programs can be very fast, since they can have direct access to the screen. Each user has a large degree of autonomy and can
allocate his workstation’s resources as he sees fit. Local disks add to this independence, and make it possible to continue working to a lesser or greater degree even in the face of file server crashes.. For example, if a task is started every 20 msec and runs for 10 msec each time, it uses up 0.5 CPUs. Five such tasks would need three CPUs to do the job. A set of tasks that meets the foregoing requirement is said to beschedulable.Note that the equation above really gives a lower bound on the number of CPUs needed, since it ignores task switching time, message transport, and other sources of overhead, and assumes that optimal scheduling is possible.. The interesting case is that of replicated read-write pages. In many DSM systems, when a process tries to read a remote page, a local copy is made because the system does not know what is on the page or whether it is writable. Both the local copy (in fact, all copies) and the original page are set up in their respective MMUs as read only. As long as all references are reads, everything is fine.. [Картинка: any2fbimgloader163]. Finally, the process descriptor also contains a descriptor for each thread in the process. The content of a thread descriptor is architecture dependent, but as a bare minimum, it contains the thread’s program counter and stack pointer. It may also contain additional information necessary to run the thread, including other registers, the thread’s state, and various flags. Brand new processes contain only one thread in their process descriptors, but stunned processes may have created additional threads before being stunned.. In contrast, Mach’s communication is from process to port, rather than from process to process. Furthermore, the sender and port must be on the same node. Using a network message server or the NORMA code as a proxy, communication can be extended over a network, but this indirection extracts a performance penalty. Mach does not support group communication or reliable broadcasting as basic kernel primitives.. Communication is done using themach_msg system call, which has seven parameters, ten options, and 35 potential error messages. It supports both synchronous and asynchronous message passing. This approach is the antithesis of the Amoeba strategy of “Keep it simple and make it fast.” The idea here is to provide the maximum flexibility and the widest range of support for present and future applications..