tailieunhanh - distributed operating system phần 3

Tham khảo tài liệu 'distributed operating system phần 3', công nghệ thông tin, đồ họa - thiết kế - flash phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SEC. CLOCK SYNCHRONIZATION 119 mutual exclusion and election algorithms. After that we will study a high-level synchronization technique called atomic transactions. Finally we will look at deadlock in distributed systems. . CLOCK SYNCHRONIZATION Synchronization in distributed systems is more complicated than in centralized ones because the former have to use distributed algorithms. It is usually not possible or desirable to collect all the information about the system in one place and then let some process examine it and make a decision as is done in the centralized case. In general distributed algorithms have the following properties 1. The relevant information is scattered among multiple machines. 2. Processes make decisions based only on local information. 3. A single point of failure in the system should be avoided. 4. No common clock or other precise global time source exists. The first three points all say that it is unacceptable to collect all the information in a single place for processing. For example to do resource allocation assigning I O devices in a deadlock-free way it is generally not acceptable to send all the requests to a single manager process which examines them all and grants or denies requests based on information in its tables. In a large system such a solution puts a heavy burden on that one process. Furthermore having a single point of failure like this makes the system unreliable. Ideally a distributed system should be more reliable than the individual machines. If one goes down the rest should be able to continue to function. Having the failure of one machine . the resource allocator bring a large number of other machines its customers to a grinding halt is the last thing we want. Achieving synchronization without centralization requires doing things in a different way from traditional operating systems. The last point in the list is also crucial. In a centralized system time is unambiguous. When a process wants to know the .