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2.1.7. The Application Layer. As we pointed out earlier, this particular example is a stateless server. For a UNIX-like server, one would have additional proceduresopen andclose, and different parameters forread andwrite. The concept of RPC itself is neutral, permitting the system designers to build any kind of servers they desire.. Networks that do not have multicasting sometimes still havebroadcasting, which means that packets containing a certain address (e.g., 0) are delivered to all machines. Broadcasting can also be used to implement groups, but it is less efficient. Each machine receives each broadcast, so its software must check to see if the packet is intended for it. If not, the packet is discarded, but some time is wasted processing the interrupt. Nevertheless, it still takes only one packet to reach all the members of a group.. 21. When a computation runs for a long time, it is sometimes wise to make checkpoints periodically, that is, to save the state of the process on disk in case it crashes. In that way, the process can be restarted from the check point instead of from the beginning. Try to devise a way of checkpointing a computation that consists of multiple processes running in parallel.. Finally, although we are not explicitly discussing the subject here, in a multiprocessor system, it is actually possible for the threads in a single address space to run in parallel, on different CPUs. This is, in fact, one of the major ways in which sharing is done on such systems. On the other hand, a properly designed program that uses threads should work equally well on a single CPU that timeshares the threads or on a true multiprocessor, so the software issues are pretty much the same either way.. A files can haveattributes,which are pieces of information about the file but which are not part of the file itself. Typical attributes are the owner, size, creation date, and access permissions. The file service usually provides primitives to read and write some of the attributes. For example, it may be possible to change the access permissions but not the size (other than by appending data to the file). In a few advanced systems, it may be possible to create and manipulate user-defined attributes in addition to the standard ones.. They are also relatively cheap, although more expensive than videotape. The first two properties are the same as videotape, but the third opens the following possibility. Imagine a file server with ann–gigabyte file system in main memory, and ann–gigabyte optical disk as backup. When a file is created, it is stored in main memory and marked as not yet backed up. All accesses are done using main memory. When the workload is low, files that are not yet backed up are transferred to the optical disk in the background, with bytek in memory going to bytek on the disk. Like the first scheme, what we have here is a main memory file server, but with a more convenient backup device having a one-to-one mapping with the memory.. The next step along the path toward distributed shared memory systems are ring-based multiprocessors, exemplified byMemnet(Delp, 1988; Delp et al., 1991; and Tarn et al., 1990). In Memnet, a single address space is divided into a private part and a shared part. The private part is divided up into regions so that each machine has a piece for its stacks and other unshared data and code. The shared part is common to all machines (and distributed over them) and is kept consistent by a hardware protocol roughly similar to those used on bus-based multiprocessors. Shared memory is divided into 32-byte blocks, which is the unit in which transfers between machines take place.. 6.5. SHARED-VARIABLE DISTRIBUTED SHARED MEMORY. [Картинка: any2fbimgloader203]. Another directory inpublic ishosts, which contains a directory for each machine in the system. This directory contains capabilities for various servers that can be found on a host, such as a disk server, a terminal server, a process server, a random number server, and so on. Finally,pool contains capabilities for the pool processors, grouped by CPU architecture. A mechanism is present to restrict each user to a specific set of pool processors.The Directory Server Interface. Fig. 8-20. Intermachine communication in Mach proceeds in five steps.. Interruptsare caused by asynchronous events, such as clock ticks or the completion of an I/O request. They are not necessarily related to anything the current thread is doing, so it is not possible to let that thread’s process handle them. Instead, it is possible to arrange in advance that when an interrupt occurs on a certain interrupt vector (i.e., a specific device), a new kernel thread will be created spontaneously to process it. If a second interrupt occurs on the same vector before the first one has terminated, a second thread is created, and so on. All I/O interrupts except the clock are handled this way. The clock is fielded by the supervisor itself, but it can be set up to notify a user thread if desired. Interrupt threads can invoke only a limited set of kernel services because the system is in an unknown state when they are started. All they can do are operations on semaphores and mutexes, or send minimessages to special miniports.. Time servers come in two varieties, local and global. The local ones participate in timekeeping within their cells. The global ones keep local time servers in different cells synchronized. The time servers communicate among themselves periodically to keep their clocks consistent. They also use the algorithm of Fig. 10-17 for choosing the new UTC.. In this example, we generate and use a new session key at each step. This much paranoia is not required, but the protocol allows it and doing so allows very short lifetimes for each key if the clocks are well synchronized..