8-core iMac Pro, using stockfish (chess engine) to selectively utilise 1-16 cores. Using a chess engine allows one to see how much output one is getting in terms of problem solving, as this does not necessarily scale linearly with the clock speed (or indeed the number of cores the engine is using).
First of all, using only 10-20 second short tests where CPU temperature does not get a chance to rise above 80C. Using Instruments.app to alter processor cores, and Intel Power Gadget.app to measure CPU speeds and temps.
(a) if on default out-of-the-box 16 active processor cores with hyperthreading on, then it makes no difference whether Stockfish runs on 1, 2, 3 ... all the way up to 16 cores, the clock speed is 3.90GHz in all instances.
(b) If I ask Stockfish to use just 1 core ...
(i) and turn down the CPU to 1 active core without hyperthreading, then the clock speed goes up to 4.2GHz.
(ii) CPU is on 2 cores, still 4.2GHz
(iii) CPU 3 cores, 4.0GHz
(iii) CPU 4 cores, averaging 3.98GHz
(iv) CPU 5 cores, 3.92GHz
(v) CPU 6-8 cores, 3.90GHz
Please note that Stockfish actually runs at least 10% FASTER as single core on 16-core CPU at 3.9GHz, than it does on 1-core CPU at 4.2GHz, presumably due to interruption by other processes. "Efficient" execution of single-core Stockfish, i.e. where delivers chess as fast as on an out-of-the-box iMP, is only reached when 4 or more CPU cores are active.
Now at longer execution times on out-of-the-box iMP,
(a) using 16 Stockfish cores, when CPU temp hits 91-93C (which is fairly soon, within a minute or two), then the CPU starts to throttle and speed jumps to and fro between 3.63 and 3.90GHz
(b) using 8 Stockfish cores, even though CPU temp VERY gradually rises to 93C NO throttling occurs, and it continues to run at 3.90GHz. I watched this for about 15 minutes and it was flat-lining the curves at that point, steady state.
Note that Stockfish runs FASTER at 8 cores than it does at 16 cores even before throttling sets in: hyperthreading 16 cores is less efficient than running 8 unhyperthreaded cores.
