%0 Journal Article
%@ 0163-5999
%A Casale, Giuliano
%A Tribastone, Mirco
%D 2013
%F eprints:2604
%I ACM
%J Performance Evaluation Review
%N 4
%P 73-82
%T Modelling exogenous variability in cloud deployments
%U http://eprints.imtlucca.it/2604/
%V 40
%X Describing exogenous variability in the resources used by a cloud application leads to stochastic performance models that are difficult to solve. In this paper, we describe the blending algorithm, a novel approximation for queueing network models immersed in a random environment. Random environments are Markov chain-based descriptions of timevarying operational conditions that evolve independently of the system state, therefore they are natural descriptors for exogenous variability in a cloud deployment. The algorithm adopts the principle of solving a separate transient-analysis subproblem for each state of the random environment. Each subproblem is then approximated by a system of ordinary differential equations formulated according to a fluid limit theorem, making the approach scalable and computationally inexpensive. A validation study on several hundred models shows that blending can save up to two orders of magnitude of computational time compared to simulation, enabling efficient exploration of a decision space, which is useful in particular at design-time.