An Objective Test of a Mac's Cooling System Condition
  • There was a Mac Mini thread in which someone seemed to be having issues with Security Spy related to the machine overheating and throttling the CPU. As you know, the CPU internally slows itself down if die temperatures gets too high. This especially means the processor stops Turobo boosting. The effective clock speed drops when the processor gets too hot.

    As a computer gets dust in its fans, radiators and upon its components, and undergoes thermal paste degradation, the cooling system becomes less and less effective. Personally, I frequently needed to replace thermal paste when a machine gets to about 4-5 years old. Wantonly replacing thermal paste has some risk, So, it is best down when you know the paste is no longer working correctly.

    One could guess at how things are doing, but there is a neat way to quick check whether the cooling system is still able to keep up with maximal CPU loads and avoid throttling.

    1. Download “Intel Power Gadget” from Intel and install that on your machine. Intel Power Gadget displays the current CPU clock frequency as well as its spec clock speed. You can see on a graphical display when the CPU goes over the spec frequency during turboboost. Turboboost lets your CPU run faster than spec while there is thermal headroom.

    2. Open a terminal window run
    yes > /dev/null

    That will max out 1 core. Watch how the CPU frequency rises to above spec. That’s turboboost in effect.

    3. Open another terminal window and also “yes > /dev/null” in that terminal window.

    You now have 2 cores maxed out.

    Keep opening another new terminal window and running an instance of “yes > /dev/null”

    On a 4 core i5, four instances will max out the CPU. On a 4 core i7, it will take 8 instance for the CPU to becomes fully saturated with work. (Activity Monitor will show almost zero CPU idle)

    As you adde more instance of yes > /dev/null and increase CPU load, watch how the temperature rises and whether the CPU frequency drops back down (throttling).

    The processor will drop its clock frequency if it gets too hot.

    On a properly working Mac Mini 4 core i7, you should be able to max out the CPU (8 instances) and still avoid throttling. The fans will be maxed out and the temperature in the 99-102C range, but you should NOT see the frequency drop back down to spec. If you see throttling happening, your Mac’s cooling system needs cleaning and possible new thermal paste.

    Now you have a simple, objective test for whether your CPU might be throttling down when under a heavy Security Spy load. --- All without physically tearing apart your Mac to inspect its thermal paste.
  • BTW, quit Terminal to end all those yes processes.
  • Forgot to mention....

    Also keep track of the fan speed when the CPU's are maxed out. The fan speed will be higher for the same CPU load when your thermal paste is past its best service life (about four years) in our hot running Macs.

    For instance, on my 13 inch dual core i7 MacBook Pro, replacing the stock paste with GElid GC Extreme change the fan speed at max CPU load down from 5469 RPM to 3253 RPM. The machine was able to run at max load without throttling on the old paste, but only if it spun up its fans to very high RPM.
  • Great tutorial - many thanks for posting!

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