Bus Fig. 1-5. A bus-based multiprocessor.. Communication in Distributed Systems. In this design, the “sequential process” model that we had in the first two cases is lost. The state of the computation must be explicitly saved and restored in the table for every message sent and received. In effect, we are simulating the threads and their stacks the hard way. The process is being operated as a finite-state machine that gets an event and then reacts to it, depending on what is in it.. The goals of the scheduler activation work are to mimic the functionality of kernel threads, but with the better performance and greater flexibility usually associated with threads packages implemented in user space. In particular, user threads should not have to be make special nonblocking system calls or check in advance if it is safe to make certain system calls. Nevertheless, when a thread blocks on a system call or on a page fault, it should be possible to run other threads within the same process, if any are ready.. In some systems, there is no distinction between clients and servers. All machines run the same basic software, so any machine wanting to offer file service to the public is free to do so. Offering file service is just a matter of exporting the names of selected directories so that other machines can access them.. [Картинка: any2fbimgloader131]. 3. No data access (read or write) is allowed to be performed until all previous accesses to synchronization variables have been performed.. For this reason, an additional mechanism is often needed for synchronization. One possibility is a synchronization manager (or managers) that accept messages asking to enter and leave critical regions, lock and unlock variables, and so on, sending back replies when the work is done. When a region cannot be entered or a variable cannot be locked, no reply is sent back immediately, causing the sender to block. When the region becomes available or the variable can be locked, a message is sent back. In this way, synchronization can be done with a minimum of network traffic, but at the expense of centralizing control per lock.. Deadlocks are impossible because even if two processes try to update the same object at the same time, one of them will get to the primary copy first to lock it, and the other request will be queued until the object is free again. Also note that during the update process, all copies of the object are locked, so no other processes can read the old value. This locking guarantees that all operations are executed in a globally unique sequential order.. 2. Is it really essential that Dash make a distinction between the CLEAN and UNCACHED states? Would it have been possible to dispense with one of them? After all, in both cases memory has an up-to-date copy of the block.. Although in practiceRequest for Status messages are rare, they do occur, and thus raise the mean number of messages required for a reliable broadcast slightly above 2, even when there are no lost messages. The effect increases slightly as the number of machines grows.. 8.1.1. History of Mach. 9.5.3. Implementation of UNIX on Chorus. [Картинка: any2fbimgloader235]. 10. A DCE programmer needs to time a benchmark program. It calls the local clock time procedure (on its own machine) and learns that the time is 15:30:00.0000. Then it starts the benchmark immediately. When the benchmark finishes, it calls the time procedure again and learns that the time is now 15:35:00.0000. Since it used the same clock, on the same machine, for both measurements, is it safe to conclude that the benchmark ran for 5 minutes (±0.1 msec)? Defend your answer..