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/* Definition of the message format. */. (a). 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.. Fourth and last, the entire RPC system has been carefully coded in assembly language and hand optimized. This last point is probably the reason that the various components in Fig. 2-27 are as uniform as they are. No doubt when the measurements were first made, they were more skewed, prompting the authors to attack the most time consuming parts until they no longer stuck out.. As soon as multiple CPUs are introduced, each with its own clock, the situation changes. Although the frequency at which a crystal oscillator runs is usually fairly stable, it is impossible to guarantee that the crystals in different computers all run at exactly the same frequency. In practice, when a system hasn computers, alln crystals will run at slightly different rates, causing the (software) clocks gradually to get out of sync and give different values when read out. This difference in time values is calledclock skew.As a consequence of this clock skew, programs that expect the time associated with a file, object, process, or message to be correct and independent of the machine on which it was generated (i.e., which clock it used) can fail, as we saw in themake example above.. 4.2.4. A Hybrid Model. Another implementation issue is how overhead is dealt with. Many theoretical processor allocation algorithms ignore the overhead of collecting measurements and moving processes around. If an algorithm discovers that by moving a newly created process to a distant machine it can improve system performance by 10 percent, it may be better to do nothing, since the cost of moving the process may eat up all the gain. A proper algorithm should take into account the CPU time, memory usage, and network bandwidth consumed by the processor allocation algorithm itself. Few do, mostly because it is not easy.. Still, NFS has been widely criticized for not implementing the proper UNIX semantics. A write to a file on one client may or may not be seen when another client reads the file, depending on the timing. Furthermore, when a file is created, it may not be visible to the outside world for as much as 30 sec. Similar problems exist as well.. The unifying

concept behind Linda is that of an abstracttuple space. The tuple space is global to the entire system, and processes on any machine can insert tuples into the tuple space or remove tuples from the tuple space without regard to how or where they are stored. To the user, the tuple space looks like a big, global shared memory, as we saw in Fig. 6-35. The actual implementation may involve multiple servers on multiple machines, and will be described later.. The Orca system consists of the language, compiler, and runtime system, which actually manages the shared objects during execution. Although language, compiler, and runtime system were designed to work together, the runtime system is independent of the compiler and could be used for other languages as well. After an introduction to the Orca language, we will describe how the runtime system implements an object-based distributed shared memory. The Orca Language. 13. In Fig. 6-14, is 000000 a legal output for a memory that is only PRAM consistent? Explain your answer.. In method 2, the full message appears only once on the network, plus a very shortAccept message from the sequencer, so only half the bandwidth is consumed. On the other hand, every machine is interrupted twice, once for the message and once for theAccept. Thus method 1 wastes bandwidth to reduce interrupts compared to method 2. Depending on the average message size, one may be preferable to the other.. 9.1.5. Kernel Structure. Each message contains a header (for the microkernel’s internal use only), an optional fixed part and an optional body. The header identifies the source and destination and contains various protection identifiers and flags. The fixed part, if present, is always 64 bytes long and is entirely under user control. The body is variable sized, with a maximum of 64K bytes, and also entirely under user control. From the kernel’s point of view, both the fixed part and the body are untyped byte arrays in the sense that the kernel does not care what is in them.. Chorus has been designed to handle real-time applications, with or without UNIX. The Chorus scheduler, in particular, reflects this goal. Priorities range from 1 to 255. The lower the number, the higher the priority, as in UNIX..

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