Cpu frequency stays at the middle

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jshan
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Joined: Sat Jan 25, 2020 9:51 am

Cpu frequency stays at the middle

Post by jshan »

Hello,

My problem is the following:

Cpu normally stays at around 3.5 GHz. When I start some computing (e.g. Python script doing some ai) all 8 cpus go to max (in htop). After a few seconds, cpu freq goes to 2.1 for all the cores, even though htop still shows 100% usage of all 8 cores.

I suspect that at some point intel scaling governor prevents overheating by setting clock to 2.1 instead of max. Fan also slows down when this happens.

What I tried to set freq back to max:
- Changing governor from power save to performance. No change.
- Ac on or battery - no change
- Updated to latest os 19.1
- Disabled intel_pstate in grub, and using userspace cpu scaling applet. This changed something, but it went worse. More governors appeared in the applet and frequency selection between 800-1800MHz appeared. None of those options worked and cpu freq fell down to 1.4GHz. So I decided to return back to intel_pstate on - at least cpu goes to 2.1GHz.

Setup:
Thinkpad E490, i5 8265 1.6-3.9GHz.

I am open to new experiments to boost this i5 a while.
Please advice me how to show this intel driver that it could boost the fan a little bit instead of lowering cpu.

Thanks.

SpookyWatcher
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Re: Cpu frequency stays at the middle

Post by SpookyWatcher »

This sounds like thermal throttling. Do you have temps that are associated with the cpu @ 3.5ghz and when it drops to 2.1ghz?

You can install Psensor to watch temps as you work if need.

jshan
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Re: Cpu frequency stays at the middle

Post by jshan »

Thank you for the hint. It turned out that this throttling happens at 70 degrees. After some research - this is found in some lenovo laptops and is known as "lenovo throttling problem".
Fix (partial) is available in github, for some of the affected laptops (including mine). Fix (according to me) is actually that throttling temperature is moved from 70 to 85 degrees. This helps using the cpu for longer time until that temperature is hit. Then, the throttling lowers frequency to ~2.4 (which is better than previous 2.1).

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smurphos
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Re: Cpu frequency stays at the middle

Post by smurphos »

You might be interested in some reading on the p-state driver - https://www.kernel.org/doc/html/v4.12/a ... state.html

The key info is that on these chips the Turbo frequencies (anything > 1.6Ghz on the i5-8265U) aren't sustainable indefinitely. On my machine ( a Dell laptop with a i5-8265U ) under sustained load (e.g video encoding) it tops out at 2.1 or 2.2 Ghz. I only see frequencies at the top end of the range very briefly when doing other short CPU intensive tasks.
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