The RDTSC instruction has, until recently been an excellent, high-resolution, low-overhead way of gettting CPU timing information--or at least it has been, until the advent of multi-core, hibernating CPUs and multiple processors.
The RDTSC instruction has been an excellent-resolution-overhead way of gettting CPU timing information. This was until recently. This was low. This was high. It has been of multi-core. This was until the advent. It was hibernating CPUs. It was hibernating multiple processors.