Join-the-Shortest-Queue (JSQ) is a very old and popular routing policy for server farms. Figure 1 shows two examples of server farm architectures employing JSQ routing. In both cases, each incoming job is immediately dispatched, via a front-end router, to the queue with the fewest number of jobs, designated as the shortest queue (ties are broken at random). In Figure 1(a), jobs at a queue are served in First-Come-First-Served (FCFS) order. In Figure 1(b), jobs within a queue are served according to Processor-Sharing (PS), meaning that when there are n jobs at a queue, they share the processing capacity, each simultaneously receiving 1/nth of the service. We refer to Figure 1(a) as a JSQ/FCFS server farm and to Figure 1(b) as a JSQ/PS farm. If more detail is needed, we use the notation: M/G/K/JSQ/PS, denoting a Poisson arrival process, i.i.d. job sizes from a general distribution, K servers, JSQ routing; and PS scheduling at queues.
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