Task Manager is LYING About Your CPU Usage (Here's the Truth)

A deep dive into how Windows Task Manager actually measures CPU usage, written by the original Task Manager author Dave Plummer. The post explains that Task Manager uses cumulative process execution time deltas rather than wall-clock intervals as its denominator, making measurements robust against timer imprecision. Key insights include: why a single-threaded process maxing one core on an 8-core machine shows ~12% instead of 100%, how the idle process acts as an accounting bucket for unused cycles, why the display is a time-averaged window rather than instantaneous, and how modern dynamic CPU frequency scaling means time-based accounting no longer reflects actual work done. Also covers historical quirks like the 99% CPU cap (which hid a rare kernel accounting bug), PID reuse edge cases, and WOW (Windows on Windows) 16-bit task attribution.