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Fig. 2-13. (a) Individually acknowledged messages. (b) Reply being used as the acknowledgement of the request. Note that the ACKs are handled entirely within the kernels.. The idea behind RPC is to make a remote procedure call look as much as possible like a local one. In other words, we want RPC to be transparent— the calling procedure should not be aware that the called procedure is executing on a different machine, or vice versa. Suppose that a program needs to read some data from a file. The programmer puts a call toread in the code to get the data. In a traditional (single-processor) system, theread routine is extracted from the library by the linker and inserted into the object program. It is a short procedure, usually written in assembly language, that puts the parameters in registers and then issues a READ system call by trapping to the kernel. In essence, theread procedure is a kind of interface between the user code and the operating system.. Fig. 3-7. (a) The time daemon asks all the other machines for their clock values. (b) The machines answer. (c) The time daemon tells everyone how to adjust their clock.. In summary, event-triggered designs give faster response at low load but more overhead and chance of failure at high load. Time-trigger designs have the opposite properties and are furthermore only suitable in a relatively static environment in which a great deal is known about system behavior in advance. Which one is better depends on the application. In any event, we note that there is much lively controversy over this subject in real-time circles.Predictability. However, first, a few words of warning about these and similar measurements are in order. Some of the measurements are static, meaning that they represent a snapshot of the system at a certain instant. Static measurements are made by examining the disk to see what is on it. These measurements include the distribution of file sizes, the distribution of file types, and the amount of storage occupied by files of various types and sizes. Other measurements are dynamic, made by modifying the file system to record all operations to a log for subsequent analysis. These data yield information about the relative frequency of various operations, the number of files open at any moment, and the amount of sharing that takes place. By combining the static and dynamic measurements, even though they are fundamentally different, we can get a better picture of how the file system is used.. In essence, this design is similar to traditional virtual memory systems: when a process touches a nonresident page, a trap occurs and the operating system fetches the page and maps it in. The difference here is that instead of getting the page from the disk, the operating system gets it from another processor over the network. To the user processes, however,

the system looks very much like a traditional multiprocessor, with multiple processes free to read and write the shared memory at will. All communication and synchronization can be done via the memory, with no communication visible to the user processes. In effect, Li and Hudak devised a system that is both easy to program (logically shared memory) and easy to build (no physically shared memory).. A problem arises if the requesting machine has no free space in its cache to hold the incoming block. To make space, it picks a cached block at random and sends it home, thus freeing up a cache slot. Blocks whoseHome bit are set are never chosen since they are already home.. The kernel manages five principal abstractions:. Theexception portis used to report exceptions caused by the process. Typical exceptions are division by zero and illegal instruction executed. The port tells the system where the exception message should be sent. Debuggers also use the exception port.. Various implementations of C threads are available on Mach. The original one did everything in user space inside a single process. This approach timeshared all the C threads over one kernel thread, as shown in Fig. 8-5(a). This approach can also be used on UNIX or any other system that provides no kernel support. The threads were run as coroutines, which means that they were scheduled nonpreemptively. A thread could keep the CPU as long as it wanted or was able to. For the producer-consumer problem, the producer would eventually fill the buffer and then block, giving the consumer a chance to run. For other applications, however, threads had to callyield from time to time to give other threads a chance.. Fig. 8-9. Three processes sharing a mapped file.. A UNIX process runs as a Chorus user process, on top of the UNIX subsystem. As such, it has all the features of a Chorus process, although existing UNIX programs will not use all of them. The standard text, data, and stack segments are implemented by three mapped Chorus segments.. The object manager is needed only on machines containing a disk, so on a diskless workstation the configuration of Fig. 9-22(b) would be most appropriate. When a process reads or writes a file on this machine, the process manager forwards the request to the object manager on the user’s file server. In principle, either the user’s machine or the file server can do caching, but not both, except for segments that have been opened or mapped in read-only mode.. 11. Give a possible use of port migration in Chorus.. Fig. 10-13. The steps in writing a client and a server..

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