DiagSim: Systematically Diagnosing Simulators for Healthy Simulations.

Simulators are the most popular and useful tool to study computer architecture and examine new ideas. However, modern simulators have become prohibitively complex (e.g., 200K+ lines of code) to fully understand and utilize. Users therefore end up analyzing and modifying only the modules of interest (e.g., branch predictor, register file) when performing simulations. Unfortunately, hidden details and inter-module interactions of simulators create discrepancies between the expected and actual module behaviors. Consequently, the effect of modifying the target module may be amplified or masked and the users get inaccurate insights from expensive simulations. In this article, we propose DiagSim, an efficient and systematic method to diagnose simulators. It ensures the target modules behave as expected to perform simulation in a healthy (i.e., accurate and correct) way. DiagSim is efficient in that it quickly pinpoints the modules showing discrepancies and guides the users to inspect the behavior without investigating the whole simulator. DiagSim is systematic in that it hierarchically tests the modules to guarantee the integrity of individual diagnosis and always provide reliable results. We construct DiagSim based on generic category-based diagnosis ideas to encourage easy expansion of the diagnosis. We diagnose three popular open source simulators and discover hidden details including implicitly reserved resources, un-documented latency factors, and hard-coded module parameter values. We observe that these factors have large performance impacts (up to 156%) and illustrate that our diagnosis can correctly detect and eliminate them.