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Our last category consists of switched multicomputers. Various interconnection networks have been proposed and built, but all have the property that each CPU has direct and exclusive access to its own, private memory. Figure 1-8 shows two popular topologies, a grid and a hypercube. Grids are easy to understand and lay out on printed circuit boards. They are best suited to problems that have an inherent two-dimensional nature, such as graph theory or vision (e.g., robot eyes or analyzing photographs).. Finally, we come to the hardest one,parallelism transparency.In principle, a distributed system is supposed to appear to the users as a traditional, uniprocessor timesharing system. What happens if a programmer knows that his distributed system has 1000 CPUs and he wants to use a substantial fraction of them for a chess program that evaluates boards in parallel? The theoretical answer is that together the compiler, runtime system, and operating system should be able to figure out how to take advantage of this potential parallelism without the programmer even knowing it. Unfortunately, the current state-of-the-art is nowhere near allowing this to happen. Programmers who actually want to use multiple CPUs for a single problem will have to program this explicitly, at least for the foreseeable future. Parallelism transparency can be regarded as the holy grail for distributed systems designers. When that has been achieved, the work will have been completed, and it will be time to move on to new fields.. At first glance, mailboxes appear to eliminate the race conditions caused by messages being discarded and clients giving up. However, mailboxes are finite and can fill up. When a message arrives for a mailbox that is full, the kernel once again is confronted with the choice of either keeping it around for a while, hoping that at least one message will be extracted from the mailbox in time, or discarding it. These are precisely the same choices we had in the unbuffered case. Although we have perhaps reduced the probability of trouble, we have not eliminated it, and have not even managed to change its nature.. 1. BEGIN_TRANSACTION: Mark the start of a transaction.. Some people make a distinction between two kinds of distributed deadlocks: communication deadlocks and resource deadlocks. A communication deadlock occurs, for example, when processA is trying to send a message to processB, which in turn is trying to send one to process C, which is trying to send one toA. There are various scenarios in which this situation leads to deadlock, such as no buffers being available. A resource deadlock occurs when processes are fighting over exclusive access to I/O devices, files, locks, or other resources.. Due to the relatively greater cost of creating and destroying threads in the kernel, some systems take an environmentally correct approach and recycle their threads. When a thread is destroyed, it is marked as not runnable, but its kernel data structures are not otherwise affected. Later, when a new thread must be created, an old thread is reactivated, saving some overhead. Thread recycling is also possible for user-level threads, but since the thread management overhead is much smaller, there is less incentive to do this.. Task 1 starts up tasks 2 and 3 on its machine, as well as task 7 on processorB. Each of these three tasks starts up another task, and so on, as illustrated. The arrows indicate messages being sent between tasks. In this simple example, it is perhaps easiest to think ofX->Y as meaning thatY cannot start until a message fromX has arrived. Some tasks, such as 8, require two messages before they may start. The cycle is completed when task 10 has run and generated the expected response to the initial stimulus.. Each CPU does its caching independent of the others. Consequently, it is possible for a particular word to be cached at two or more CPUs at the same time. Now let us consider what happens when a write is done. If no CPU has the word being written in its cache, the memory is just updated, as if caching were not being used. This operation requires a normal bus cycle. If the CPU doing the write has the only copy of the word, its cache is updated and memory is updated over the bus as well.. Fortunately, for many actual programs, once a CPU has written a word, that CPU is likely to need the word again, and it is unlikely that another CPU will use the word quickly. This situation suggests that if the CPU using the word could somehow be given temporary “ownership” of the word, it could avoid having to update memory on subsequent writes until a different CPU exhibited interest in the word. Such cache protocols exist. Goodman (1983) devised the first one, calledwrite once.However, this protocol was designed to work with an existing bus and was therefore more complicated than is strictly necessary. Below we will describe a simplified version of it, which is typical of all ownership protocols. Other protocols are described and compared by Archibald and Baer(1986).. Group communication also uses FLIP. When a message is sent to multiple destinations, FLIP uses the hardware multicast or broadcast on those networks where it is available. On those that do not have it, broadcast is simulated by sending individual messages, just as we saw on the wide-area network. The choice of mechanism is done by the FLIP layer, with the same user semantics in all cases.. CallDescriptionCreateCreate a new directoryDeleteDelete a directory or an entry in a directoryAppendAdd a new directory entry to a specified directoryReplaceReplace a single directory entryLookupReturn the capability set corresponding to a specified nameGetmasksReturn the rights masks for the specified entryChmodChange the rights bits in an existing directory entry. The directory server is a critical component in the Amoeba system, so it has been implemented in a fault-tolerant way. The basic data structure is an array of capability pairs stored on a raw disk partition. This array does not use the bullet server because it must be updated frequently and the overhead was thought to be too much.. 9. Why does thetrans call have parameters for both sending and receiving? Would it not have been better and simpler to have two calls,send_requestandget_reply, one for sending and one for receiving?. As of 1988, the Mach 2.5 kernel was large and monolithic, due to the presence of a large amount of Berkeley UNIX code in the kernel. In 1988, CMU removed all the Berkeley code from the kernel and put it in user space. What remained was a microkernel consisting of pure Mach. In this chapter, we will focus on the Mach 3 microkernel and one user-level operating system emulator, for BSD UNIX. One difficulty, however, is that Mach is under development, so any description is at best a snapshot. Fortunately, most of the basic ideas discussed in this chapter are relatively stable, but some of the details may change in time.. The port management calls are shown in Fig. 9-17. The first four are straightforward, allowing ports to be created, destroyed, enabled, and disabled. The last one specifies a port and a process. After the call completes, the port no longer belongs to its original owner (which need not be the caller) but instead belongs to the target process. It alone can now read messages from the port..