2. Some memory management.. 10. In all the examples in this chapter, a server can only listen to a single address. In practice, it is sometimes convenient for a server to listen to multiple addresses at the same time, for example, if the same process performs a set of closely related services that have been assigned separate addresses. Invent a scheme by which this goal can be accomplished.. (c) Remote banking.. These properties are often referred to by their initial letters,ACID.. The method has several major advantages over conventional RPC. First, threads do not have to block waiting for new work. Thus no context has to be saved. Second, creating a new thread is cheaper than restoring an existing one, since no context has to be restored. Finally, time is saved by not having to copy incoming messages to a buffer within a server thread. Various other techniques can also be used to reduce the overhead. All in all, a substantial gain in speed is possible.. Although Ferguson et al. do not go into the details, such an economic model raises all kinds of interesting questions, among them the following. Where do processes get money to bid? Do they get regular salaries? Does everyone get the same monthly salary, or do deans get more than professors, who in turn get more than students? If new users are introduced into the system without a corresponding increase in resources, do prices get bid up (inflation)? Can processors form cartels to gouge users? Are users’ unions allowed? Is disk space also chargeable? How about laser printer output? The list goes on and on.. Fig. 4-21. Triple modular redundancy.. 5.2. DISTRIBUTED FILE SYSTEM IMPLEMENTATION. To guard against the situation that the primary crashes before it has had a chance to instruct all the secondaries, the update should be written to stable storage prior to changing the primary copy. In this way, when a server reboots after a crash, a check can be made to see if any updates were in progress at the time of the crash. If so, they can still be carried out. Sooner or later, all the secondaries will be updated.. When studying client caching, we saw that a large fraction of the trouble is caused by the fact that if two clients are caching the same file and one of them modifies it, the other does not discover this, which leads to inconsistencies. A little thought will reveal that this situation is highly analogous to memory caches in a multiprocessor. Only there, when one processor modifies a shared word, a hardware signal is sent over the memory bus to the other caches to allow them to invalidate or update that word. With distributed file systems, this is not done.. Also important is the expected change in applications. Most experimental distributed systems being built at universities focus on programming in a UNIX-like environment as the canonical application, because that is what the researchers themselves do all day (at least when they are not in committee meetings or writing grant proposals). Initial data suggest that not all 50 million French citizens are going to list C programming as their primary activity. As distributed systems become more widespread, we are likely to see a shift to electronic mail, electronic banking, accessing data bases, and recreational activities, which will change file usage, access patterns, and a great deal more in ways we as yet do not know.. So far, so good. The trouble arises when a CPU wants to write a word that two or more CPUs have in their caches. If the word is currently in the cache of the CPU doing the write, the cache entry is updated. Whether it is or not, it is also written to the bus to update memory. All the other caches see the write (because they are snooping on the bus) and check to see if they are also holding the word being modified. If so, they invalidate their cache entries, so that after the write completes, memory is up-to-date and only one machine has the word in its cache.. Once astack has been defined, variables of this type can be declared, as in. 10.3.1. Goals of DCE RPC. Before a client can call a server, it has to locate the server and bind to it. Naive users can ignore the binding process and let the stubs take care of it automatically, but binding happens nevertheless. Sophisticated users can control it in detail, for example, to select a specific server in a particular distant cell. In this section we will describe how binding works in DCE..