I am using QueryPerformanceCounter to time some code. I was shocked when the code starting reporting times that were clearly wrong. To convert the results of QPC into \"real\"
You should always expect the core frequency to change on any CPU that supports technology such as SpeedStep or Cool'n'Quiet. Wall time is not affected, it uses a RTC. You should probably stop using the performance counters, unless you can tolerate a few (5-50) millisecond's worth of occasional phase adjustments, and are willing to perform some math in order to perform the said phase adjustment by continuously or periodically re-normalizing your performance counter values based on the reported performance counter frequency and on RTC low-resolution time (you can do this on-demand, or asynchronously from a high-resolution timer, depending on your application's ultimate needs.)