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In the long term, the main driving force will be the existence of large numbers of personal computers and the need for people to work together and share information in a convenient way without being bothered by geography or the physical distribution of people, data, and machines.. Since there is only one memory, if CPUA writes a word to memory and then CPUВ reads that word back a microsecond later,В will get the value just written. A memory that has this property is said to becoherent.Coherence plays an important role in distributed operating systems in a variety of ways that we will study later.. 1. An interprocess communication mechanism.. [Картинка: any2fbimgloader33]. Given this background, now consider how the client locates the server. When the client calls one of the remote procedures for the first time, say,read, the client stub sees that it is not yet bound to a server, so it sends a message to the binder asking toimportversion 3.1 of of thefile_server interface. The binder checks to see if one or more servers have already exported an interface with this name and version number. If no currently running server is willing to support this interface, theread call fails. By including the version number in the matching process, the binder can ensure that clients using obsolete interfaces will fail to locate a server rather than locate one and get unpredictable results due to incorrect parameters.. The exact list of primitives depends on what kinds of objects are being used in the transaction. In a mail system, there might be primitives to send, receive, and forward mail. In an accounting system, they might be quite different. READ and WRITE are typical examples, however. Ordinary statements, procedure calls, and so on, are also allowed inside a transaction.. Since a file server is normally just a user process (or sometimes a kernel process) running on some machine, a system may contain multiple file servers, each offering a different file service. For example, a distributed system may have two servers that offer UNIX file service and MS-DOS file service, respectively, with each user process using the one appropriate for it. In that way, it is possible to have a terminal with multiple windows, with UNIX programs running in some windows and MS-DOS programs running in other windows, with no conflicts. Whether the servers offer specific file services, such as UNIX or MS-DOS, or more general file services is up to the system designers. The type and number of file services available may even change as the system evolves.. A completely different approach to the semantics of file sharing in a distributed system is to make all files immutable. There is thus no way to open a file for writing. In effect, the only operations on files are CREATE and READ.. Another important component of the Amoeba configuration consists of specialized servers, such as file servers, which for hardware or software reasons need to run on a separate processor. In some cases a server is able to run on a pool processor, being started up as needed, but for performance reasons it is better to have it running all the time.. The second form of interthread communication is the mutex. Amutexis like a binary semaphore. It can be in one of two states, locked or unlocked. Trying to lock an unlocked mutex causes it to become locked. The calling thread continues. Trying to lock a mutex that is already locked causes the calling thread to block until another thread unlocks the mutex. If more than one thread is waiting on a mutex, when the mutex is unlocked, exactly one thread is released. In addition to the calls to lock and unlock mutexes, there is also one that tries to lock a mutex, but if it is unable to do so within a specified interval, times out and returns an error code. Mutexes are fair and respect thread priorities.. A process can also unmap segments. Furthermore, a process can specify a range of virtual addresses and request that the range be unmapped, after which those addresses are no longer legal. When a segment or a range of addresses is unmapped, a capability is returned so the segment may still be accessed, or even mapped back in again later, possibly at a different virtual address.. Theyield call is a hint to the scheduler that the thread has nothing useful to do at the moment, and is waiting for some event to happen before it can continue. An intelligent scheduler will take the hint and run another thread. In Mach, which normally schedules its threads preemptively,yield is only optimization. In systems that have nonpreemptive scheduling, it is essential that a thread that has no work to do release the CPU, to give other threads a chance to run.. It is worth noting that the paging daemon is part of the kernel. Although the page replacement algorithm is completely machine independent, with a memory full of pages owned by different memory managers, there is no suitable way to let one of them decide which page to evict. The only method that might be possible is to statically partition the page frames among the various managers and let each one do page replacement on its set. However, since global algorithms are generally more efficient than local ones, this approach was not taken. Subsequent work has investigated this subject (Harty and Cheriton, 1992; and Subramian, 1991).. It is worth noting that when the kernel sends a message to a memory manager, it is effectively making an upcall. Although flexibility is gained this way, some system designers consider it inelegant for the kernel to call user programs to perform services for it. These people usually believe in hierarchical systems, with the lower layers providing services to the upper layers, not vice versa.. All of these calls operate by first determining whether CDS or GDS is needed. X.500 names are handled by GDS; DNS or mixed names are handled by CDS, as illustrated in Fig. 10-24. First let us trace the lookup of a name in X.500 format. The XDS library sees that it needs to look up an X.500 name, so it calls theDUA (Directory User Agent), a library linked into the client code. This handles GDS caching, analogous to the CDS clerk, which handles CDS caching. Users have more control over GDS caching than they do over CDS caching and can, for example, specify which items are to be cached. They can even bypass the DUA if it is absolutely essential to get the latest data..