I installed Ubuntu 12.04 on Macbook Pro 8.1 and one of the first things I noticed was that the fans were starting to spin very fast every few minutes for 10-30 sec and then going back to normal. That was happening even without any processor load, when completely idle. The fans were usually spinning at >4000 RPM and made much noise. The computer was not getting hotter than usual. When running OSX Lion there was no noise at all, fans almost all the time at 2000 RPM.
I spent some time on it and found out that Precise uses a deamon to control the temperature, called macfanctld. You can use /etc/macfanctld.conf to set the configuration.
I found out that the high fan speed is not due to the fact that the temperature is getting hot, but because there are two sensors which indicate wrong numbers (you can check that using 'sensors' command ):
or setting the macfanctld log level to 2:
Speed: 4992, *AVG: 56.9C, TC0P: 50.2C, TG0P: 51.5C, Sensors: TB0T:34 TB1T:34 TB2T:33 TC0C:58 TC0D:56 TC0E:59 TC0F:60 TC0P:50 TC1C:58 TC2C:58 TC3C:58 TC4C:57 TCFC:0 TCGC:57 TCSA:53 TCTD:256 TG0D:52 TG0P:52 THSP:42 TM0S:64 TMBS:0 TP0P:54 TPCD:60 TW0P:129 Th1H:51 Th2H:48 Tm0P:40 Ts0P:32 Ts0S:43
Moreover, TCTD was randomly jumping from temperatures of 0 to 256, so this may be the reason for unjustified random fan speeds.
macfanctld is taking an average of the sensors including the values above, so the actual AVG temp used to control the fans is wrong, usually biased up, hence high RPM and noise.
The workaround solution is to use an option in the macfanctld.conf which allows to ignore the malfunctioning sensors:
exclude: 13 16 21 24
After reboot the reported temperatures are usually normal and the fans are working at reasonable speeds. I tested the response of the fans to heavy processor load by asking MATLAB to invert 10000x10000 matrix and the AVG temperature jumped to 63deg, and the fan to max 6200 RPM and then got it back to normal temperature. So I think it is safe so far.
There is a expired bug about the failing sensor readings: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/955538 which may be good to open again.
My question would be: does anyone know what the failing sensors do and if there is any danger in excluding them? Maybe some better solution to this problem?