1. A model of a software system under execution comprising: a representation of a first node corresponding to a first function executing in the software system which obtains access to a resource, with an indicator of a number of times the first function has been called in the software system; a representation of a second node corresponding to a second function executing in the software system while the resource is accessed, with an indicator of a number of times the second function has been called; a representation of a place holder node with an indicator that the place holder node has never been called in the software system; a representation of a first call from the first node to the place holder node; a representation of a second call from the place holder node to the second node; and wherein the first and second calls correspond to the first function calling the second function in the software system.