The Intel Xeon 6208U CPU is advertised to have a base frequency of 2.9 GHz and Turbo Boost frequency of 3.9 GHz - but that is for one core, which is not very useful (and IMO, highly misleading). I'm interested in the Turbo Boost frequency for all cores. Intel now intentionally hides this information but cpu-monkey.com shows it to be 2.6 Ghz. Is this possible? What does it mean for the Turbo Boost frequency to be lower than the base frequency?
SnapOverflow
It depends on two things, for the most part: whether you're using AVX-512, and how hot the CPU gets when under a sustained load.
If you keep the CPU's reported temperature below its rated Tcase temperature, and you're not using AVX-512, it'll run at 2.9GHz on an all-core load all day long. The moment you execute an AVX-512 instruction, it'll drop to 2.6GHz.
AVX-512 execution is handled by dedicated parts of the CPU silicon that don't do anything else; they produce a ludricrous amount of heat, and they can't run at quite the same speed as the rest of the chip. As a result, every Intel CPU with AVX-512 lowers clocks by a few hundred MHz when it's in use.
The good news is that there's really not that much outside of specialized/HPC workloads that ever touches AVX-512, so you'll probably never notice :)