1. A method for performing capacity planning for applications running on a computational infrastructure, wherein the method comprises: instrumenting an application under development to receive one or more performance metrics under a physical deployment plan, wherein said instrumenting is carried out via a module executing on a hardware processor; receiving the one or more performance metrics from the computational infrastructure hosting one or more applications that are currently running, wherein said receiving is carried out via a module executing on a hardware processor; using a predictive inference engine to determine how the application under development can be deployed, wherein said predictive inference engine predicts performance of the application under development on top of the one or more applications currently running on the computational infrastructure under multiple hypothetical computational infrastructure deployment plans, and wherein said using a predictive engine is carried out via a module executing on a hardware processor; and using the determination to perform capacity planning for the applications on the computational infrastructure by identifying a computational infrastructure deployment plan from the multiple hypothetical computational infrastructure deployment plans that incurs no quality of service (QoS) violations for the application under development or the one or more applications currently running on the computational infrastructure, wherein said using the determination is carried out via a module executing on a hardware processor.