1. A method of finding costly interactions among components of a computer system, said method comprising: a) defining a graph having nodes which represent said components and having edges which represent interactions among said components; b) measuring costs of said system such that each path in said graph has an associated cost; c) defining a path-based interface, which is independent of any system and any method for measuring said costs, said interface providing operations to generate summaries which represent costs for associated paths in said graph and providing operations to query any of said summaries for its costs and associated paths; and d) using the operations provided by the path-based interface to find costly summaries, whose associated paths represent costly interactions among components of the system.