Ubuntu – Fans aren’t fullspeed while CPU is 100°, then CPU frequency decrease


Since I upgraded to from 12.04 to 16.04, my Dell XPS 13 (dev edition/Sputnik) laptop seems very slow (and less noisy) to me. Before upgrrading, under heavy load, I was able to hear the fans going full-speed all the time. Now, the same load, the fans are fast at the beginning but then become slower.

I'm using tool i7z to measure my core i7 Intel processor data.

I'm measuring the fans speed by ear in this test.

Load test

Phase 1

Before producing load. The frequency is high, in C7 state, and the fans are slow.

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Phase 2

At the beginning of the load phase, the CPU frequency remains high, the fans are running fast, and the temperature increase

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Phase 3

After like 30 seconds of heavy load, the CPU frequency remains high, the fans slow down for unexpected reasons, and the temperature continue to increase (probably because the fans slowed down)

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Phase 4

After some seconds at very high temperature (100-105°C, my CPU junction temp is 105), something decrease significantly the frequency and the temperature returns to an acceptable state. The fans are still slow.

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Phase 5

The load is stopped, the frequency goes up again, and I can even sometimes hear the fan speed increase.

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Can someone tell me what is happening here? Why would my fans become slow while my CPU is around 105° and become fast again when I stop the load and the temperature is around 75°?

Note that I can't open my laptop now (don't have the tooling) to clean the fans. But I suspect it's not a dust problem because the fans are able to run at full-speed, they just aren't running full-speed all the required time. Also the fans used to run much longer at full-speed under Ubuntu 12.04.

Also note that I disabled thermald and cpufreqd.

I have tried editing grub to use both acpi-cpufreq and intel_pstate drivers. The only notable difference is that on phase 4 above, the frequency will decrease to around 1Ghz for intel_pstate while acpi-cpufreq will only decrease frequency to around 1.6Ghz

It seems Dell laptop use i8kmon to control fan speed.

During the phase 3 above (while my fans have been slowed down), if I run i8kfan 2 2. The fan speed can increase (according to sensors it increase from 6000 RPM to 7300 RPM). This increase only last for some seconds and after that the fans go down to 6000 RPM again.

Related issues I opened to solve my problem (may contain additional infos not mentionned here that could be useful)


Actually I tried clean formatted installs of Ubuntu 12.04 / 14.04 / 16.04 and did not get better result with any of those.

Could not install Sputnik ISO for weird reasons.

My Bios is A10 which seems the latest for my computer.

On boot I run Dell diagnostic tool and got the following:

Fan problem

(I made sure to give back control of fans to bios using sudo ./smm 30a3 because I disabled this)

This is a bit weird because I can clearly hear my fan speed goes up and down according to load so there must be something in control of it but I don't know what.

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