The CPU has an overhead of about ~10us to enable the AVX512 units.
It also dramatically reduces the clock on other cores.
For more information, see: https://stackoverflow.com/a/56861355
Timing information: https://www.agner.org/optimize/microarchitecture.pdf
Its all about AMD Zen4 or Xeon Ice Lake+, which has no clock reduction and no overheads.
On Alder Lake (pg 172).
> The reader is referred to the timings for Tiger Lake and Gracemont.
On Tiger Lake (pg 167):
> Warm-up period for ZMM vector instructions
> The processor puts the upper parts of the 512 bit vector execution units into a low power mode when they are not used.
> Instructions with 512-bit vectors have a throughput that is approximately 4.5 times slower than normal during an initial warm-up period of approximately 50,000 clock cycles.
I'm not saying you are wrong. I just haven't heard about that.
> Since 512-bit instructions are reusing the same 256-bit hardware, 512-bit does not come with additional thermal issues. There is no artificial throttling like on Intel chips.
At least for Zen4, there's no worries about throttling or anything really. Its the same AVX hardware, "double pumped" (two 256-bit micro-instructions output per single 512-bit instruction). But you still save significantly on the decoder (ie: the "other" hyperthread can use the decoder in the core to keep executing its scalar code at full speed, since your hyperthread is barely executing any instructions)