A batch-job computer facility with a single CPU opens its doors at 7 A.M. and closes its doors at midnight, but operates until all jobs present at midnight have been processed. Assume that jobs arrive at the facility with interarrival times that are exponentially distributed with mean 1.91 minutes. Jobs request either express (class 4), normal (class 3), deferred (class 2), or convenience (class 1) service; and the classes occur with respective probabilities 0.05, 0.50, 0.30, and 0.15. When the CPU is idle, it will process the highest-class (priority) job present, the rule’s being FIFO within a class. The times required for the CPU to process class 4, 3, 2, and 1 jobs are 3-Erlang random variables (see Sec. 2.7) with respective means 0.25, 1.00, 1.50, and 3.00 minutes. Simulate the computer facility for each of the following cases:
(a) A job being processed by the CPU is not preempted by an arriving job of a higher class.
(b) If a job of class i is being processed and a job of class j (where j > i) arrives, the arriving job preempts the job being processed. The preempted job joins the queue and takes the highest priority in its class, and only its remaining service time needs to be completed at some future time.
Estimate for each class the expected time-average number of jobs in queue and the expected average delay in queue. Also estimate the expected proportion of time that the CPU is busy and the expected proportion of CPU busy time spent on each class. Note that it is convenient to have one list for each class’s queue and also an input parameter that is set to 0 for case (a) and 1 for case (b). Use stream 1 for the interarrival times, stream 2 for the job-class determination, and streams 3, 4, 5, and 6 for the processing times for classes 4, 3, 2, and 1, respectively.
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