That is when the cpu is allocated to a user process

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Unformatted text preview: less than n. This is because when multiple processors cooperate to execute the programs or subprograms, a certain amount of overhead is incurred in keeping everything working correctly. This overhead, plus contention for shared resources, lowers the expected gains from additional processors. 2. Better Reliability. Due to multiplicity of processors, multiprocessor systems also have better reliability than single-processor systems. In a properly designed multiprocessor system, if one of the processors breaks down, the other processor(s) automatically takes over the system workload until repairs are made. Thus a complete breakdown of such systems can be avoided. For example, if a system has 4 processors and one fails, then the remaining 3 processors can be utilized to process the jobs submitted to the system. Thus, the entire system runs only 25% slower, rather than failing altogether. This ability of a system to continue providing service proportional to the level of non-failed hardware is called graceful degradation feature. Multiprocessing systems, however, require a very sophisticated operating system to schedule, balance, and coordinate the input, output, and processing activities of multiple processors. The design of such an operating system is a complex and time taking job. Moreover, multiprocessing systems are expensive to procure and maintain. In addition to the high charge paid initially, the regular operation and maintenance of these systems is also a costly affair. Time-sharing Time-sharing is a mechanism to provide simultaneous interactive use of a computer system by many users in such a way that each user is given the impression that he/she has his/her own computer. It uses multiprogramming with a special CPU scheduling algorithm to achieve this. A time-sharing system has many (even hundreds of) user terminals simultaneously connected to the same computer. Using these terminals, multiple users can simultaneously work on the system. The multiprogramming feature allows multiple user programs to simultaneously reside in the main memory. The special CPU scheduling algorithm us...
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This document was uploaded on 04/07/2014.

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