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As a final example, think about a large bank with hundreds of branch offices all over the world. Each office has a master computer to store local accounts and handle local transactions. In addition, each computer has the ability to talk to all other branch computers and with a central computer at headquarters. If transactions can be done without regard to where a customer or account is, and the users do not notice any difference between this system and the old centralized mainframe that it replaced, it too would be considered a distributed system.. A different switch design copies the cell into a queue associated with the output buffer and lets it wait there, instead of keeping it in the input buffer. This approach eliminates head-of-line blocking and gives better performance. It is also possible for a switch to have a pool of buffers that can be used for both input and output buffering. Still another possibility is to buffer on the input side, but allow the second or third cell in line to be switched, even if the first one cannot be..  long dest; /* receiver’s identity */. 7. If the communication primitives in a client-server system are nonblocking, a call tosend will complete before the message has actually been sent. To reduce overhead, some systems do not copy the data to the kernel, but transmit it directly from user space. For such a system, devise two ways in which the sender can be told that the transmission has been completed and the buffer can be reused.. All the algorithms have the same underlying model of the system, which we will now describe. Each machine is assumed to have a timer that causes an interruptHtimes a second. When this timer goes off, the interrupt handler adds 1 to a software clock that keeps track of the number of ticks (interrupts) since some agreed-upon time in the past. Let us call the value of this clockC.More specifically, when the UTC time ist,the value of the clock on machinepisCp(t).In a perfect world, we would haveCp(t)=tfor allpand allt.In other words,dC/dtideally should be 1.. 4.1.1. Introduction to Threads. Kernel threads do not require any new, nonblocking system calls, nor do they lead to deadlocks when spin locks are used. In addition, if one thread in a process causes a page fault, the kernel can easily run another thread while waiting for the required page to be brought in from the disk (or network). Their main disadvantage is that the cost of a system call is substantial, so if thread operations (creation, deletion, synchronization, etc.) are common, much more overhead will be incurred.. One last twist that is sometimes present in a distributed system but rarely in a centralized one is the possibility of looking up an ASCII name and getting notone butseveral binary names (i-nodes, capabilities, or something else). These would typically represent the original file and all its backups. Armed with multiple binary names, it is then possible to try to locate one of the corresponding files, and if that one is unavailable for any reason, to try one of the others. This method provides a degree of fault tolerance through redundancy.. The most straightforward place to store all files is on the server’s disk. There is generally plenty of space there and the files are then accessible to all clients. Furthermore, with only one copy of each file, no consistency problems arise.. In addition toout andin, Linda also has a primitiveread, which is the same asin except that it does not remove the tuple from the tuple space. There is also a primitiveeval, which causes its parameters to be evaluated in parallel and the resulting tuple to be deposited in the tuple space. This mechanism can be used to perform an arbitrary computation. This is how parallel processes are created in Linda.. Finally, the process descriptor also contains a descriptor for each thread in the process. The content of a thread descriptor is architecture dependent, but as a bare minimum, it contains the thread’s program counter and stack pointer. It may also contain additional information necessary to run the thread, including other registers, the thread’s state, and various flags. Brand new processes contain only one thread in their process descriptors, but stunned processes may have created additional threads before being stunned.. 7.5.3. The Fast Local Internet Protocol. When a message comes in to the UNIX server, an idle thread accepts it, determines which process it came from, extracts the system call number and parameters from it, carries it out, and finally, sends back the reply. Most messages correspond exactly to one BSD system call.. Fig. 9-13. Selected calls relating to segments.. Condition variablesprovide a second synchronization mechanism. These are used in conjunction with mutexes. Typically, when a thread needs some resource, it uses a mutex to gain exclusive access to a data structure that keeps track of the status of the resource. If the resource is not available, the thread waits on a condition variable, which atomically suspends the thread and releases the mutex. Later, when another thread signals the condition variable, the waiting thread is restarted..