Thanks for your answer - great blog link - most relevant:
"Intel made more aggressive use of AVX512 instructions in earlier versions of the icc compiler, but has since removed most use unless the user asks for it with a special command line option.".
For other readers, short summary copied from the link with very light editing:
* There are heavy and light AVX instructions. "Heavy" instructions roughly are those involving floating point operations or integer multiplications operating on 512 bits.
* Intel cores can run in one of three modes: license 0 (L0) [normal], license 1 (L1) is slower, and license 2 (L2) is the slowest. To get into license 2, you need sustained use of heavy 512-bit instructions, where sustained means approximately one such instruction every cycle.
* The processor does not immediately move to a higher license when encountering heavy instructions: it will first execute these instructions with reduced performance (say 4x slower) and only when there are many of them will the processor change its frequency. Otherwise, any other 512-bit instructions will move the core to L1.
* Downclocking is per core and for a short time after you have used particular instructions (e.g., ~2ms).
* The downclocking of a core is based on: the current license level of that core, and also the total number of active cores on the same CPU socket (irrespective of the license level of the other cores).
The constraints and suggested workarounds are complex. Also the word choice "license" by Intel is bizarre (since it implies the throttling is to reduce performance for business reasons rather than technical reasons?).
"Intel made more aggressive use of AVX512 instructions in earlier versions of the icc compiler, but has since removed most use unless the user asks for it with a special command line option.".
For other readers, short summary copied from the link with very light editing:
* There are heavy and light AVX instructions. "Heavy" instructions roughly are those involving floating point operations or integer multiplications operating on 512 bits.
* Intel cores can run in one of three modes: license 0 (L0) [normal], license 1 (L1) is slower, and license 2 (L2) is the slowest. To get into license 2, you need sustained use of heavy 512-bit instructions, where sustained means approximately one such instruction every cycle.
* The processor does not immediately move to a higher license when encountering heavy instructions: it will first execute these instructions with reduced performance (say 4x slower) and only when there are many of them will the processor change its frequency. Otherwise, any other 512-bit instructions will move the core to L1.
* Downclocking is per core and for a short time after you have used particular instructions (e.g., ~2ms).
* The downclocking of a core is based on: the current license level of that core, and also the total number of active cores on the same CPU socket (irrespective of the license level of the other cores).
The constraints and suggested workarounds are complex. Also the word choice "license" by Intel is bizarre (since it implies the throttling is to reduce performance for business reasons rather than technical reasons?).