RE: apparently random fan "kick-ins"...
Hi all,
The fans's speeds are controlled by an algorithm that depends upon the internal temperatures of the various sensors within the rMBP. I've never been able to ascertain precisely the algorithm, but it is certainly true that whenever the CPU temperatures (and it depends on which ones) go above 90C (=194F) then Apple's fan control algorithm will increase the speed of the fans.
You have to be cautious when reading internal temperature with the various temperature monitor tools, as different ones will report different temperatures. How is this possible, you might ask? Well, there are actually a number of temperature sensors, and since the apps typically only report one number, they are either just choosing a single sensor to report or are performing some sort of "averaging" of the many temperature sensors's values. If you wish to see really accurate values, I'd recommend Intel's own PCM tools for providing complete processor performance information. (Unfortunately, you have to compile these Intel PCM tools yourself on your computer and they run from the command line, although there might be a GUI interface to them that I don't know about.)
About those apparently random fan "kick-ins" that last roughly 30 seconds....well, I suspect that they are caused by various system processes, and most likely the Spotlight indexing of your drives. If you attach an external hard drive to your computer that has not yet been Spotlight indexed, then Spotlight will attempt to index it. This takes CPU cycles and will load a CPU and heat it up, eventually causing the fans to spin up to a high speed. Note that Spotlight indexing runs at a higher "nice" value, i.e., it runs at a lower priority, than your interactive processes, thus once you start to use your computer the indexing takes a backseat to your interactive processes and slows down, not utilizing the same CPU load as before you began to interact with your computer. In summary, because of Spotlight indexing your computer will apparently, at random and when you are not doing anything with it, ramp up the fans's speeds. You can check if it is Spotlight indexing by looking in the Activity Monitor to see if you have "mds" and "mdworker" processes running -- these are the Spotlight indexing processes.
There are other system processes besides just Spotlight whose running can also cause your fans to speed up their spin rates. These will also occur at apparently random times when you are not actively using your computer.
Regards,
Switon
