1.2.1. Advantages of Distributed Systems over Centralized Systems. In some systems, another option is available: do not let a process send a message if there is no room to store it at the destination. To make this scheme work, the sender must block until an acknowledgement comes back saying that the message has been received. If the mailbox is full, the sender can be backed up and retroactively suspended as though the scheduler had decided to suspend it justbefore it tried to send the message. When space becomes available in the mailbox, the sender is allowed to try again.. Conceptually, when a process starts a transaction, it is given a private workspace containing all the files (and other objects) to which it has access. Until the transaction either commits or aborts, all of its reads and writes go to the private workspace, rather than the “real” one, by which we mean the normal file system. This observation leads directly to the first implementation method: actually giving a process a private workspace at the instant it begins a transaction.. Now consider the situation of Fig. 3-26, where we are going to allow preemption. Given that our system believes in ancestor worship, as we discussed above, we do not want a young whippersnapper preempting a venerable old sage, so Fig. 3-26(a) and not Fig. 3-26(b) is labeledpreempt. We can now safely label Fig. 3-26(b)wait. This algorithm is known aswound-wait,because one transaction is supposedly wounded (it is actually killed) and the other waits. It is unlikely that this algorithm will make it to the Nomenclature Hall of Fame.. Although scheduler activations solve the problem of how to pass control to an unblocked thread in a process one of whose threads has just blocked, it creates a new problem. The new problem is that an interrupted thread might have been executing a semaphore operation at the time it was suspended, in which case it would probably be holding a lock on the ready list. If the runtime system started by the upcall then tries to acquire this lock itself, in order to put a newly ready thread on the list, it will fail to acquire the lock and a deadlock will ensue. The problem can be solved by keeping track of when threads are or are not in critical regions, but the solution is complicated and hardly elegant.. When a new message comes in to the server’s machine, the kernel creates a new thread on-the-fly to service the request. Furthermore, it maps the message into the server’s address space, and sets up the new thread’s stack to access the message. This scheme is sometimes calledimplicit receiveand it is in contrast to a conventional thread making a system call to receive a message. The thread that is created spontaneously to handle an incoming RPC is occasionally referred to as apop-up thread.The idea is illustrated in fig. 4-9.. An analytical queueing model of this algorithm has been constructed and investigated. Using this model, it was established that the algorithm behaves well and is stable under a wide range of parameters, including different threshold values, transfer costs, and probe limits.. A simple but practical way to build a multiprocessor is to base it on a bus to which more than one CPU is connected. Fig. 6-2(a) illustrates a system with three CPUs and a memory shared among all of them. When any of the CPUs wants to read a word from the memory, it puts the address of the word it wants on the bus and asserts (puts a signal on) a bus control line indicating that it wants to do a read. When the memory has fetched the requested word, it puts the word on the bus and asserts another control line to announce that it is ready. The CPU then reads in the word. Writes work in an analogous way.. 6.3.2. Sequential Consistency. 21. Give an example of anin operation in Linda that does not require any searching or hashing to find a match.. 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.. Although this method of running UNIX programs looks cumbersome, measurements have shown that it compares favorably with traditional monolithic kernel implementations (Golub et al., 1990). Future work will focus on splitting the UNIX server into multiple servers with more specific functions. Eventually, the single UNIX server may be eliminated, although this depends on how the work with multiple servers develops during the course of time.. Fig. 9-22. Different applications may best be handled using different configurations.. The traditional cryptographic model is shown in Fig. 10-25. The client has an unencrypted message,P, called theplaintext, which is transformed by an encryption algorithm parametrized by a key,K. The encryption can be done by the client, the operating system, or by special hardware. The resulting message,C, called theciphertextis unintelligible to anyone not possessing the key. When the ciphertext arrives at the server, it is decrypted usingK, which results in the original plaintext. The notation generally used to indicate encryption is. TheBasic Overseer Serverruns on every server machine. Its job is to make sure that the other servers are alive and well. If it discovers that some servers have crashed, it brings up new versions. It also provides an interface for system administrators to stop and start servers manually..