What is the benefit of equipping a computer or smartphone with multiple CPU cores that differ in clock speed? [closed]

Question

I've bought a samsung galaxy s21 ultra about a year ago. Today i was looking at the specs again and i noticed that the Samsung Exynos 2100 processor consists out of 1 single CPU core of 2.9 GHz, three cores of 2.8 Ghz and 4 cores of 2.2 ghz. On the Samsung Exynos 2100 info page I read that the 2.8GHz Triple-core (Cortex®-A78) is added to improve the performance of programs that make use of AI and ML.

So some of these cores seem to specialize in processing specific tasks. But i do wonder why they don't all have the same clock speed. Wouldn't that make multithreading more efficient, for instance when all 8 cores are used to process a complex computer task? Because now you have some cores that will take longer to finish a chunk of the computer task than the other cores.

Answer 1

You mix power eating high performance cores with power saving low performance cores. That way you have minimal power consumption most of the time, and high performance when you need it. Eight high performance cores instead of 4+4 would empty your battery in half the time, and make the device so hot that the clock rate needs to be slowed down quickly. And eight high performance cores are bloody expensive.

BTW. I tested this on an old iPhone XR (2 + 4 cores). Using six threads, it finished all threads at practically exactly the same time. The operating system is easily capable of moving threads between cores, so they all make the same progress. The people creating iOS are not stupid, and I suppose the people creating Android aren't stupid either.

PS. It's much more than just clock speed. On Apple devices, the low power cores have much less L1 and L2 cache, fewer execution units (3 instead of 9), much fewer out of order execution capabilities. That's why their performance is nowhere near the performance cores, but neither is their power consumption. Interestingly, some widely used benchmark hasn't figured out that there are two different numbers for L2 cache (much bigger for performance cores than for low power cores), and they display the much smaller number.