Massive performance loss under xen: cpufreq issue

[Patrick Plenefisch <simonpatp@xxxxxxxxx>]

Hi,

I am having issues with performance in xen, to the tune of 20-30%!

I first noticed this when doing a kernel compile that seemed to take slightly longer than expected, then in a benchmark that was 25% slower. I also noticed that the max power draw was slightly lower, and the idle power draw was slightly higher

In native linux, `cpufreq-info` shows that most cpus idle between 0.8 and 1.4GHz, and run with loads at 4.8GHz

In dom0 linux under xen, `xenpm get-cpufreq-states` shows that most cpus idle at 2.2GHz, and run with loads at 4.2GHz

I tried adjusting the governor, but xen was still capped at 4.2GHz

I have a Threadripper 7960X on a ASRock TRX50 WS, and I can reproduce this in xen 4.14, 4.17, and 4.18, and with Linux kernels 5.10 and 6.1. The latter is from a brand-new debian 12 install.

What configuration or tuning should I look at to better utilize the frequency range?

Thanks,

Patrick