Selected Examples

Servers that Break Down II

Another variant of the server breakdown model concerns what happens to the transaction that is in service when the breakdown occurs. In the model in Figure 9.14, even though the server is stopped when it breaks, the transaction in service completes service. The model in Figure 9.15 adds the pre-emption of the transaction in service, which is routed back into the queue.

Figure 9.15: A Server that Breaks Down

By default, the transaction is placed at the end of the FIFO queue. So, if there were other transactions waiting for service, the pre-empted transaction would be behind them. Another variant on this model would place the pre-empted transaction into the front of the queue even though the queue was a FIFO for nonpre-empted transactions. This variant could be accomplished using a priority queue where the transaction priority is the simulation time at the time the transaction arrived to the queue and the queue has decreasing priority (see Figure 2.5). See the preceding example on priority queues.

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