C P Us Clock Frequency

When i in my career, c p us always ran at a fixed frequency. To day, it seems like clock frequency can vary dramatically over time. What challenges does this pose for timing or tracing operations? And do real c p us ind data centers do the frequency? Is it variable? Or do they tend to lock it down to something, fearing the clock? Frequency is technique for reducing power consumption and therefore heat generation. It first started with intail speed step in the eighties.