Notes About Accuracy

Significant disruptions in the real-time capabilities of the profiled programs must be expected, because performance profiling is performed by software (rather than by hardware with an external bus analyzer or equivalent device). Performance profiling using software slows down the processor, and the profiled applications may behave differently when being profiled compared to when running at full processor speed.

When profiling, the processor can spend more than fifty percent of the processing time profiling clock interrupts. Similarly, the time spent recording the call graph is significant, and tends to bias the profiling results in a non-linear manner.

The accuracy of the reported percentage of time spent is about five percent when the number of profiling ticks is in the order of magnitude of ten times the number of bytes in the profiled programs. In other words, in order to profile a program of 1 million bytes with any degree of accuracy, at least 10 millions ticks should be used. This level of accuracy is usually sufficient to plan code optimizations, which is the primary goal of the profiler, but the operator should beware of using all the fractional digits of the reported figures.

If more accuracy is needed, the operator can experiment with different combinations of the rate of the profiling clock, the type of profiling clock and the time spent profiling.