[SOLVED] Weird CPU scaling issues after upgrading to 20.04

[hiryu]

For whatever reason... CPU scaling in 20.04 is really weird and broken on my laptop.

In 19.10, I got pretty consistent performance. Testing with a parallel compile, I tend to get around 2.7 GHz per thread. With 20.04, I get this sometimes. Other times, the CPU's will stay around 1.2 GHz, other times at 2.2 GHz... and I had a prolonged stretch at 800 MHz. Setting the fans to run at full speed doesn't impact this issue one way or another.

Changing the CPU governor doesn't seem to make things any better or worse. Powersave and Performance seem to give the same results. I've confirmed that the changes to the CPU governor are reflected under /sys. Ie,
cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave

20.0.4 feels pretty sluggish on my laptop due to this. Possibly there's a change somewhere causing 20.04 to be less aggressive with the CPU clocks, but even so it seems odd to peg the CPU well below its rated speeds with the fans at full speed.

I tried booting back to my original 19.10 installation, the performance is definitely much better there.

I tried a custom kernel under 19.10 and 20.04 (same config), and that didn't seem impact either Kubuntu version, which makes me think it's not (necessarily) a kernel level change.

Relevant laptop specs:
Oryx Pro 5
i7 8750

[hiryu]

Worth mentioning when the system isn't loaded, the clocks stay pretty high, the clocks can quickly jump up pretty high for any little activity as you would normally expect. Once I load the CPU, the temperature is not substantial according to lm-sensors. maybe mid-50's. At idle, it tends to be in the 49-51 range. The ambient in my home isn't high either.

[hiryu]

Looks like I've solved the issue... Long story short, I found my laptop wasn't plugged into its AC adapter... It was plugged into the AC Adapter for someone else's laptop that has substantially lower power requirements than mine (lower power CPU, no discrete GPU, etc).

What's particularly interesting about this case... Is how 19.10 was seemingly unaffected by this, and I'd probably been doing this for quite some time (and I definitely was during my recent testing against 19.10). I'd argue that 20.04 was more appropriately handling this case by throttling the system given that it had insufficient power, unlike 19.10.

Kinda embarrassing, but hopefully someone Googling for a solution to the same or similar issue will stop and check what the heck their laptop is actually plugged into.

I rarely run my laptop in Nvidia GPU mode so probably I didn't push this other charger TOO hard... Hopefully.

[Valdo]

Thanks for sharing this case and unusual root cause