Complex sentence: 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.
Split into simpler sentences: 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 CPU's, and multiple processors.