Fig. 1-11. A multiprocessor with a single run queue.. All the details of the message passing are hidden away in the two library procedures, just as the details of actually making system call traps are hidden away in traditional libraries.. So far, all of this is straightforward. However, there are two issues associated with group membership that are a bit trickier. First, if a member crashes, it effectively leaves the group. The trouble is, there is no polite announcement of this fact as there is when a process leaves voluntarily. The other members have to discover this experimentally by noticing that the crashed member no longer responds to anything. Once it is certain that the crashed member is really down, it can be removed from the group.. Nevertheless, there is hope. Here is a simple algorithm that demonstrates that atomic broadcast is at least possible. The sender starts out by sending a message to all members of the group. Timers are set and retransmissions sent where necessary. When a process receives a message, if it has not yet seen this particular message, it, too, sends the message to all members of the group (again with timers and retransmissions if necessary). If it has already seen the message, this step is not necessary and the message is discarded. No matter how many machines crash or how many packets are lost, eventually all the surviving processes will get the message. Later we will describe more efficient algorithms for ensuring atomicity.Message Ordering. 3.3.2. A Ring Algorithm. System calls involving time are a problem because the clocks on different machines may not be synchronized. In Chap. 3, we saw how hard it is to achieve synchronization. Using the time on the remote machine may cause programs that depend on time, likemake, to give incorrect results. Forwarding all time-related calls back to the home machine, however, introduces delay, which also causes problems with time.. Many real-time applications and systems are highly structured, much more so than general-purpose distributed systems. Typically, an external device (possibly a clock) generates a stimulus for the computer, which must then perform certain actions before a deadline. When the required work has been completed, the system becomes idle until the next stimulus arrives.. a = 1;a = 1;b = 1;b = 1;print(b, c);b = 1;c= 1;a= 1;b = 1;print(a, c);print(a, b);c = 1;print(a, c);print(b, c);print(a, c);print(a, c); c = 1;c = 1;a = 1;print(b, c);print(a, c);print(a, b);print(b, c);print(a, b);Prints: 001011Prints: 101011Prints: 010111Prints: 111111Signature: 001011Signature: 101011Signature: 110101Signature: 111111(a)(b)(c)(d). To do aDOWN(P), it does. The rest of the information in the capability is ignored by the kernels and passed to the server for its own use. TheObject field is used by the server to identify the specific object in question. For example, a file server might manage thousands of files, with the object number being used to tell it which one is being operated on. In a sense, theObject field in a file capability is analogous to a UNIX i-node number.. Amoeba supports a simple threads model. When a process starts up, it has one thread. During execution, the process can create additional threads, and existing threads can terminate. The number of threads is therefore completely dynamic. When a new thread is created, the parameters to the call specify the procedure to run and the size of the initial stack.. There is one additional problem that must be solved. Normally, aSendToGroup terminates successfully when the sequencer has received and broadcast or approved the message. Ifk>0, this protocol is insufficient to survivek arbitrary crashes. Instead, a slightly modified version of method 2 is used. When the sequencer sees a message,M, that was just broadcast, it does not immediately broadcast anAccept message, as it does whenk=0. Instead, it waits until thek lowest-numbered kernels have acknowledged that they have seen and stored it. Only then does the sequencer broadcast theAccept message. Sincek+1 machines (including the sequencer) now are known to have storedM in their history buffers, even ifk machines crash,M will not be lost.. 7.6.6. The TCP/IP Server. Group communication provides reliable broadcasting as a user primitive. Messages can be sent to any group with a guarantee of reliable delivery. In addition, all group members see all messages arrive in exactly the same order.. 3. In DCE, some data structures are process wide, and others are per thread. Do you think that the UNIX environment variables should be process wide, or should every thread have its own environment? Defend your viewpoint..