Analysis of Randomized Join-the-Shortest-Queue (JSQ) Schemes in Large Heterogeneous Processor-Sharing Systems

In this paper, we investigate the stability and performance of randomized dynamic routing schemes for jobs based on the Join-the-Shortest Queue (JSQ) criterion in a heterogeneous system of many parallel servers. In particular, we consider servers that use processor sharing but with different server rates, and jobs are routed to the server with the smallest occupancy among a finite number of randomly sampled servers. We focus on the case of two servers that is often referred to as a Power-of-Two scheme. We first show that in the heterogeneous setting, uniform sampling of servers can cause a loss in the stability region and thus such randomized dynamic schemes need not outperform static randomized schemes in terms of mean delay in opposition to the homogeneous case of equal server speeds where the stability region is maximal and coincides with that of the static randomized routing. We explicitly characterize the stationary distributions of the server occupancies and show that the tail distribution of the server occupancy has a super-exponential behavior as in the homogeneous case as the number of servers goes to infinity. To overcome the stability issue, we show that it is possible to combine the static state-independent scheme with a randomized JSQ scheme that allows us to recover the maximal stability region combined with the benefits of JSQ, and such a scheme is preferable in terms of average delay. The techniques are based on a mean field analysis where we show that the stationary distributions coincide with those obtained under asymptotic independence of the servers and, moreover, the stationary distributions are insensitive to the job-size distribution.