Fig. 2-2. A typical message as it appears on the network.. long dest; /* receiver’s identity */. Since messages are transferred byte for byte (actually, bit for bit) over the network, the first byte sent is the first byte to arrive. In Fig. 2-20(b) we show what the message of Fig. 2-20(a) would look like if received by a SPARC, which numbers its bytes with byte 0 at the left (high-order byte) instead of at the right (low-order byte) as do all the Intel chips. When the server stub reads the parameters at addresses 0 and 4, respectively, it will find an integer equal to 83,886,080 (5×224) and a string “JILL”.. Finally, we have solution 4, expiration, in which each RPC is given a standard amount of time,T, to do the job. If it cannot finish, it must explicitly ask for another quantum, which is a nuisance. On the other hand, if after a crash the server waits a timeT before rebooting, all orphans are sure to be gone. The problem to be solved here is choosing a reasonable value ofT in the face of RPCs with wildly differing requirements.. Absolute time ordering is not always easy to implement, so some systems offer various watered-down variations. One of these isconsistent time ordering,in which if two messages, sayA andB, are sent close together in time, the system picks one of them as being “first” and delivers it to all group members, followed by the other. It may happen that the one chosen as first was not really first, but since no one knows this, the argument goes, system behavior should not depend on it. In effect, messages are guaranteed to arrive at all group members in the same order, but that order may not be the real order in which they were sent.. Various strategies are used to handle deadlocks. Four of the best-known ones are listed and discussed below.. Whereas these two strategies are approximately equally easy to implement in a multiprocessor, they differ radically in a DSM system. Unlike in a multiprocessor, where the MMU knows which word is to be written and what the new value is, in a DSM system the software does not know which word is to be written or what the new value will be. To find out, it could make a secret copy of the page about to be changed (the page number is known), make the page writable, set the hardware trap bit, which forces a trap after every instruction, and restart the faulting process. One instruction later, it catches the trap and compares the current page with the secret copy it just made, to see which word has been changed. It could then broadcast a short packet giving the address and new value on the network. The processors receiving this packet could then check to see if they have the page in question, and if so, update it.. Now let us briefly look at the algorithm for deciding whether an object should be in single-copy state or replicated. Initially, an Orca program consists of one process,
which has all the objects. When it forks, all other machines are told of this event and given current copies of all the child’s shared parameters. Each runtime system then calculates the expected cost of having each object replicated versus having it not replicated.. The second task of the kernel is to provide low-level memory management. Threads can allocate and deallocate blocks of memory, calledsegments.These segments can be read and written, and can be mapped into and out of the address space of the process to which the calling thread belongs. A process must have at least one segment, but it may also have many more of them. Segments can be used for text, data, stack, or any other purpose the process desires. The operating system does not enforce any particular pattern on segment usage.. The one-way function is in fact the same one as used for protecting capabilities, but need not be since the two concepts are unrelated.. The first version of Mach was released in 1986 for the VAX 11/784, a four-CPU multiprocessor. Shortly thereafter, ports to the IBM PC/RT and Sun 3 were done. By 1987, Mach was also running on the Encore and Sequent multiprocessors. Although Mach had networking facilities, at this time it was conceived of primarily as a single machine or multiprocessor system rather than as a transparent distributed operating system for a collection of machines on a LAN.. Copy-on-write has several advantages over doing all the copying at the time the new process is created. First, some pages are read-only, so there is no need to copy them. Second, other pages may never be referenced, so even if they are potentially writable, they do not have to be copied. Third, still other pages may be writable, but the child may deallocate them rather than using them. Here too, avoiding a copy is worthwhile. In this manner, only those pages that the child actually writes on have to be copied.. A thread in the ACTIVE state is either currently running or waiting its turn for a free CPU. In both cases it is logically unblocked and able to run. A thread in the SUSPENDED state has been suspended by another thread (or itself) that issued a kernel call asking the kernel to suspend the thread. Similarly, when a kernel call is made to stop a process, all the threads in the ACTIVE state are put in the STOPPED state until the process is released. Finally, when a thread performs a blocking operation that cannot be completed immediately, the thread is put in WAITING state until the event occurs.. 9.4. COMMUNICATON IN CHORUS. Users, machines, and other resources in a DCE system are grouped together to formcells.Naming, security, administration, and other aspects of DCE are based upon these cells. Cell boundaries usually mirror organizational units, for example, a small company or a department of a large company might be one cell..