Next generation high performance computers will massively use virtualization as a way to share hardware resources between multiple applications and to provide flexible mechanisms for fault tolerance and energy optimisation. In this context, understanding the performance behavior of virtual machines and the interference between them is a major scientific challenge that will allow a more efficient usage of resources. The first step is to characterize CPU usage sharing and to propose a performance model for virtual machines. Nonetheless, focusing on the sharing of a single CPU core is no more possible as next generation high performance machines will contain a large number of cores. Moreover, as these cores share micro-architectural resources e.g. caches, using a single core performance model is not sufficient as inter-core interference can happen. Finally, to be able to use such a model in large scale infrastructures as Clouds or high performance computers, the model must be lightweight to simulate the behavior of tens of thousands physical machines hosting hundreds of thousands virtual machines (VMs). In this paper, we present an in-depth analysis of the performance of collocated VMs. By running our experiments on the Grid'5000 testbed, we were able to evaluate 2 processor families for a total of 6 different processor models. We have systematically explored the effect of collocation by testing all the different VCPU to CPU mapping while taking into account micro-architectural components (shared caches and NUMA nodes). We also explored the effect of multi-core virtual machines. Based on these experiments, we evaluate 8 lightweight performance models and observe that the virtual machine performance can be accurately predicted using a model that takes into account the number of VMs on the core and on the related NUMA node (with less than 8% error). Finally, we validate our models on several processors and on both single and multi-(virtual)-cores VM. Using this model, we can increase the accuracy of the virtualization layer of the general purpose distributed system simulator SimGrid and improve it's usability to simulate (HPC) Clouds. These results may also be used to improve VM placement algorithms.
