ItemDescriptionSoftwareLittle software exists at present for distributed systemsNetworkingThe network can saturate or cause other problemsSecurityEasy access also applies to secret data. Finally, we come to our last and most difficult problem: How are pointers passed? The answer is: only with the greatest of difficulty, if at all. Remember that a pointer is meaningful only within the address space of the process in which it is being used. Getting back to ourread example discussed earlier, if the second parameter (the address of the buffer) happens to be 1000 on the client, one cannot just pass the number 1000 to the server and expect it to work. Address 1000 on the server might be in the middle of the program text.. An alternative design that does not suffer from this problem is thetime-triggered real-time system.In this kind of system, a clock interrupt occurs every AT milliseconds. At each clock tick (selected) sensors are sampled and (certain) actuators are driven. No interrupts occur other than clock ticks.. Earlier in this chapter, when we were discussing the semantics of distributed file systems, we pointed out that one of the design options is immutable files. One of the great attractions of an immutable file is the ability to cache it on machineA without having to worry about the possibility that machineB will change it. Changes are not permitted. Of course, a new file may have been created and bound to the same symbolic name as the cached file, but this can be checked for whenever a cached file is reopened. This model has the same RPC overhead discussed above, but the semantics are less fuzzy.. [Картинка: any2fbimgloader155]. Another issue is that a process may make thousands of consecutive writes to the same page because many programs exhibit locality of reference. Having to catch all these updates and pass them to remote machines is horrendously expensive in the absence of multiprocessor-type snooping.. An optimization of this ownership location protocol is shown in Fig. 6-28(b). Here the page manager forwards the request directly to the owner, which then replies directly back toP, saving one message.. Process creation in Amoeba is different from UNIX. The UNIX model of creating a child process by cloning the parent is inappropriate in a distributed system due to the considerable overhead of first creating a copy somewhere (FORK) and almost immediately afterward replacing the copy with a new program (EXEC). Instead, in Amoeba it is possible to create a new process on a specific processor with the intended memory image starting right at the beginning. In this one respect, process creation in Amoeba is similar to MS-DOS. However, in contrast to MS-DOS, a process can continue executing in parallel with its child, and thus can create an arbitrary number of additional children. The children can create their own children, leading to a tree of processes.. Once the coordinator has been voted into office, it collects from the other members any messages it may have missed. Now it is up to date and is able to become the new sequencer. It builds aResults message announcing itself as sequencer and telling the others what the highest sequence number is. Each member can now ask for any messages that it missed. When a member is up to date, it sends an acknowledgement back to the new sequencer. When the new sequencer has an acknowledgement from all the surviving members, it knows that all messages have been correctly delivered to the application programs in order, so it discards its history buffer, and normal operation can resume.. Before getting into the details of FLIP, it is useful to understand something about why it was designed. After all, there are plenty of existing protocols, so the invention of a new one clearly has to be justified. In Fig. 7-15 we list the principal requirements that a protocol for a distributed system should meet. First, the protocol must support both RPC and group communication efficiently. If the underlying network has hardware multicast or broadcast, as Ethernet does, for example, the protocol should use it for group communication. On the other hand, if the network does not have either of these features, group communication must still work exactly the same way, even though the implementation will have to be different.. 12. Two algorithms for broadcasting in Amoeba were presented in the text. In method 1, the sender sends a point-to-point message to the sequencer, which then broadcasts it. In method 2, the sender does the broadcast, with the sequencer then broadcasting a small acknowledgement packet. Consider a 10-Mbps network on which processing a packet-arrived interrupt takes 500 microsec, independent of the packet size. If all data packets are 1K bytes, and acknowledgement packets are 100 bytes, how much bandwidth and how much CPU time are consumed per 1000 broadcasts by the two methods?. The operations on condition variables aresignal, wait, andbroadcast, which are used to allow threads to block on a condition and later be awakened when another thread has caused that condition to occur.Implementation of C Threads in Mach. If a region has the first value, the corresponding region in the child is unallocated. References to it are treated as references to any other unallocated memory— they generate traps. The child is free to allocate the region for its own purposes or to map a memory object there.. Case Study 3: Chorus. Mapped segments are usually demand paged (unless this feature is disabled, for example, for real-time programs). When a process first references a newly mapped segment, a page fault occurs and the segment page corresponding to the address referenced is brought in and the faulting instruction restarted. In this way, ordinary virtual memory can be implemented, and in addition, a process can make one or more files visible in its virtual address space, so it can access them directly instead of having to read or write them using system calls..