Quote:
Originally Posted by Stephen R. Savage
AVX-512 only reduces performance on server chips because of artificial frequency limits that Intel imposes. After the downclock, x265 actually draws less power and runs cooler, showing how it was unnecessary. Hopefully the next generation of server improves the power management algorithm so downclocking will not be needed.
On Skylake-X/WS and Cannon Lake, AVX-512 only ever increases performance. It will presumably also be the case on Ice Lake.
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Actually the offset is needed to maintain stability. I know, because I have a 7900X, and tried to get the best out of it.
You wouldn't notice this problem with x264 or x265, because its AVX512 usage is pretty "light", but if you run some heavy AVX512 tasks on all cores, and don't configure an appropriate offset, the chip just crashes. The energy density of the AVX512 units is just too high for running at full turbo clocks, nevermind OCed.
If x265 is the only AVX512 you ever run, and you want to risk it, sure, you can disable the offset and hope that it never happens. But I prefer to know that my system is stable no matter what software does.
But be careful, and do know that you can not judge the requirement from one pretty lightweight workload.
The only way the offset is getting lower is when the cores get more efficient, which they really only do on a process shrink. So hopefully that'll significantly reduce the AVX512 offset, even if I don't expect it to go away quite just yet.
This is easily testable by anyone with such a chip. For example, recent versions of the Intel LINPACK floating-point benchmark will put enough AVX-512 stress on the CPU to cause this.