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In the general case, withn CPUs andn memories, the omega network requires log2n switching stages, each containingn/2 switches, for a total of(n log2n)/2 switches. Although for largen this is much better thann?, it is still substantial.. Operating systems cannot be put into nice, neat pigeonholes like hardware. By nature software is vague and amorphous. Still, it is more-or-less possible to distinguish two kinds of operating systems for multiple CPU systems: loosely coupled and tightly coupled. As we shall see, loosely and tightly-coupled software is roughly analogous to loosely and tightly-coupled hardware.. Finally, we come to the most important part ofheader.h, the definition of the message itself. In our example it is a structure with 10 fields. All requests from the client to the server use this format, as do all replies. In a real system, one would probably not have a fixed format message (because not all the fields are needed in all cases), but it makes the explanation simpler here. Thesource anddest fields identify the sender and receiver, respectively. Theopcode field is one of the operations defined above, that is,CREATE, READ, WRITE, orDELETE. Thecount andoffset fields are used for parameters, and two other fields,extra1 andextra2, are defined to provide space for additional parameters in case the server is expanded in the future. Theresult field is not used for client-to-server requests but holds the result value for server-to-client replies. Finally, we have two arrays. The first,name, holds the name of the file being accessed. The second,data, holds the data sent back on a reply to read or the data sent to the server on a WRITE.. 2. Ifa is the event of a message being sent by one process, andb is the event of the message being received by another process, thena->b is also true. A message cannot be received before it is sent, or even at the same time it is sent, since it takes a finite amount of time to arrive.. The log can also be used for recovering from crashes. Suppose that the process doing the transaction crashes just after having written the last log record of Fig. 3-19(d), but before changingx. After the failed machine is rebooted, the log is checked to see if any transactions were in progress at the time of the crash. When the last record is read and the current value ofx is seen to be 1, it is clear that the crash occurredbefore the update was made, sox is set to 4. If, on the other hand,x is 4 at the time of recovery, it is equally clear that the crash occurredafter the update, so nothing need be changed. Using the log, it is possible to go forward (do the transaction) or go backward (undo the transaction).Two-Phase Commit Protocol. Fig. 4-2. Per thread and per process concepts.. The research on idle workstations has centered on solving these problems. The key issues are:. A large number of processor allocation algorithms have been proposed over the years. In this section we will look at some of the key choices involved in these algorithms and point out the various trade-offs. The major decisions the designers must make can be summed up in five issues:. Although Ferguson et al. do not go into the details, such an economic model raises all kinds of interesting questions, among them the following. Where do processes get money to bid? Do they get regular salaries? Does everyone get the same monthly salary, or do deans get more than professors, who in turn get more than students? If new users are introduced into the system without a corresponding increase in resources, do prices get bid up (inflation)? Can processors form cartels to gouge users? Are users’ unions allowed? Is disk space also chargeable? How about laser printer output? The list goes on and on.. Myth 1: Real-time systems are about writing device drivers in assembly code.. Unfortunately, the NFS method makes it difficult to achieve the exact UNIX file semantics. For example, in UNIX a file can be opened and locked so that other processes cannot access it. When the file is closed, the locks are released. In a stateless server such as NFS, locks cannot be associated with open files, because the server does not know which files are open. NFS therefore needs a separate, additional mechanism to handle locking.. 1. It is more modular than the other techniques.. might describe Professor Lin in the Yale Computer Science Department. The attributes C, O, and OU are present in most names and refer to country, organization, and organization unit (department), respectively.. Each user has a secret key known only to himself and to the registry. It is computed by passing the user’s password through a one-way (i.e., noninvertible) function. Servers also have secret keys. To enhance their security, these keys are used only briefly, when a user logs in or a server is booted. After that authentication is done with tickets and PACs.. Step 4:client asks the ticket-granting server for a PAC usable by S.