Modeling of Request Cloning in Cloud Server Systems using Processor Sharing

The interest for studying server systems subject to cloned requests has recently increased. In this paper we present a model that allows us to equivalently represent a system of servers with cloned requests, as a single server. The model is very general, and we show that no assumptions on either inter-arrival or service time distributions are required, allowing for, e.g., both heterogeneity and dependencies. Further, we show that the model holds for any queuing discipline. However, we focus our attention on Processor Sharing, as the discipline has not been studied before in this context.

The key requirement that enables us to use the single server G/G/1 model is that the request clones have to receive synchronized service. We show examples of server systems fulfilling this requirement. We also use our G/G/1 model to co-design traditional load-balancing algorithms together with cloning strategies, providing well-performing and provably stable designs.

Finally, we also relax the synchronized service requirement and study the effects of non-perfect synchronization. We derive bounds for how common imperfections that occur in practice, such as arrival and cancellation delays, affect the accuracy of our model. We empirically demonstrate that the bounds are tight for small imperfections, and that our co-design method for the popular Join-Shortest-Queue (JSQ) policy can be used even under relaxed synchronization assumptions with small loss in accuracy.