Fig. 1-1. Advantages of distributed systems over centralized systems.. 1. The client is unable to locate the server.. Although in this book we will study only operating system (i.e., process) groups, it is worth mentioning that other groups are also commonly encountered in computer systems. For example, on the USENET computer network, there are hundreds of news groups, each about a specific subject. When a person sends a message to a particular news group, all members of the group receive it, even if there are tens of thousands of them. These higher-level groups usually have looser rules about who is a member, what the exact semantics of message delivery are, and so on, than do operating system groups. In most cases, this looseness is not a problem.. G=CurrentTime–MaxLifetime–MaxClockSkew. Various minor improvements are possible to this algorithm. For example, getting permission from everyone to enter a critical region is really overkill. All that is needed is a method to prevent two processes from entering the critical region at the same time. The algorithm can be modified to allow a process to enter a critical region when it has collected permission from a simple majority of the other processes, rather than from all of them. Of course, in this variation, after a process has granted permission to one process to enter a critical region, it cannot grant the same permission to another process until the first one has released that permission. Other improvements are also possible (e.g., Maekawa et al., 1987).. Deadlock detection and recovery is also popular, primarily because prevention and avoidance are so difficult. We will discuss several algorithms for deadlock detection below.. It is common for distributed systems to use both RPC and threads. Since threads were invented as a cheap alternative to standard (heavyweight) processes, it is natural that researchers would take a closer look at RPC in this context, to see if it could be made more lightweight as well. In this section we will discuss some interesting work in this area.. [Картинка: any2fbimgloader108]. Real-time systems have been around for decades, so there is a considerable amount of folk wisdom accumulated about them, most of it wrong. Stankovic (1988) has pointed out some of these myths, the worst of which are summarized here.. A node in MARS consists of at least one CPU, but often two or three work together to present the image of a single fault-tolerant, fail-silent node to the outside world. The nodes in MARS are connected by two reliable and independent TDMA broadcast networks. All packets are sent on both networks in parallel. The expected loss rate is one packet every 30 million years.. Dubois et al. (1986) define this model, calledweak consistency, by saying that it has three properties:. To summarize, object-based distributed shared memory offers three advantages over the other methods:. 14. In most implementations of (eager) release consistency in DSM systems, shared variables are synchronized on a release, but not on an acquire. Why is acquire needed at all then?. When the remote network message server gets the message, it looks up the network port number contained in it and maps it onto a local port number. In step 4, it writes the message to the local port just looked up. Finally, the server reads the message from the local port and carries out the request. The reply follows the same path in the reverse direction.. Two differences between DFS and its predecessor, AFS, are worth mentioning. In AFS, entire files were transferred, instead of 64K chunks. This strategy made a local disk essential, since files were often too large to cache in memory. With a 64K transfer unit, local disks are no longer required..