The next category is MISD, multiple instruction stream, single data stream. No known computers fit this model. Finally, comes MIMD, which essentially means a group of independent computers, each with its own program counter, program, and data. All distributed systems are MIMD, so this classification system is not tremendously useful for our purposes.. Fig. 1-9. Two clients and a server in a network operating system.. 3. What is the difference between a multiprocessor and a multicomputer?. 9. Now consider a 256-CPU hypercube. What is the worst-case delay here, again in hops?. New developments are suddenly about to make 155 Mbps the low-end standard, with major trunks running at 1 gigabit/sec and up (Catlett, 1992; Cheung, 1992; and Lyles and Swinehart, 1992). This rapid change will have an enormous impact on distributed systems, making possible all kinds of applications that were previously unthinkable, but it also brings new challenges. It is this new technology that we will describe below.. Since messages are transferred byte for byte (actually, bit for bit) over the network, the first byte sent is the first byte to arrive. In Fig. 2-20(b) we show what the message of Fig. 2-20(a) would look like if received by a SPARC, which numbers its bytes with byte 0 at the left (high-order byte) instead of at the right (low-order byte) as do all the Intel chips. When the server stub reads the parameters at addresses 0 and 4, respectively, it will find an integer equal to 83,886,080 (5×224) and a string “JILL”.. [Картинка: any2fbimgloader41]. The basic idea is that when a client wants a file, it is given aleaseon it that specifies how long the copy is valid (Gray and Cheriton, 1989). When the lease is about to expire, the client can ask for it to be renewed. If a lease expires, the cached copy may no longer be used. In this way when a client needs to read a file once, it can ask for it. When the lease expires, it just times out; there is no need to explicitly send a message telling the server that it has been purged from the cache.. Now let us consider the effect of a primary crash at various moments during an RPC. If the primary crashes before doing the work (step 2), no harm is done. The client will time out and retry. If it tries often enough, it will eventually get the backup and the work will be done exactly once. If the primary crashes after doing the work but before sending the update, when the backup takes over and the request comes in again, the work will be done a second time. If the work has side effects, this could be a problem. If the primary crashes after step 4 but before step 6, the work may end up being done three times, once by the primary, once by the backup as a result of step 3, and once after the backup becomes the primary. If requests carry identifiers, it may be possible to ensure that the work is done only twice, but getting it done exactly once is difficult to impossible.. 5.2.5. An Example: Sun’s Network File System. PROBLEMS. In this example, if processor 1 references instructions or data in chunks 0, 2, 5, or 9, the references are done locally. References to other chunks cause traps. For example, a reference to an address in chunk 10 will cause a trap to the DSM software, which then moves chunk 10 from machine 2 to machine 1, as shown in Fig. 6-25(b).. CallDescriptionCreateCreate a new file; optionally commit it as wellReadRead all or part of a specified fileSizeReturn the size of a specified fileModifyOverwrite n bytes of an uncommitted fileInsertInsert or append n bytes to an uncommitted fileDeleteDelete n bytes from an uncommitted file. [Картинка: any2fbimgloader234]. Armed with a ticket for the privilege server, the client now asks for a PAC. Unlike a ticket, which contains only the user’s login name (in ASCII), a PAC contains the user’s identity (in binary as a UUID) plus a list of all the groups to which he belongs. These are important because resources (e.g., a certain printer) are often available to all the members of certain groups (e.g., the marketing department). A PAC is proof that the user named in it belongs to the groups listed in it..