Executive SummaryThe Ampere® PMU Profiler (APP) is a Python-based tool designed to provide deep insight into the microarchitectural behavior of applications running on Ampere CPUs (e.g., Ampere® Altra® and AmpereOne®). Unlike standard profilers that identify where time is spent (e.g., which functions consume CPU time), the PMU Profiler explains why time is being spent by measuring low-level hardware events associated with the CPU pipeline and execution behavior.A key outcome of APP is that it enables performance engineers to move from coarse symptoms to actionable causes. For example, while application-level profiling can show an expensive code path, APP can help identify whether the expense stems from inefficient instruction fetching, data cache misses, or other microarchitectural factors that are difficult or impossible to isolate using application-level tools alone. The document outlines a top-down performance analysis methodology and positions APP as an essential final step for expert-level tuning, particularly on Ampere platforms, where you must understand hardware-level bottlenecks and then apply targeted code optimizations.