Stress test your CPU on Apple OS X and monitor core CPU temperatures

I’ve recently been having some kernel panics and rebooting/crashing on my MacPro 1,1 running 10.6.8 and Lion and some suggested it could be my CPUs overheating. There was a suggestion to install SMCfancontrol and run all fans and a higher speed and keep an eye on temperatures whilst stressing the CPUs to 100% for a prolonged period. So below is a little bit on info on how to crank your CPU up to 100% usage and monitor core CPU temperatures.

Getting you CPU to 100%

There are several ways to max out your CPU, some suggest ripping movies using Handbrake or re-encoding your music library but personally I don’t want all the faff with that plus these methods are IO intensive for your hard drive which isn’t good. All I do is open up Terminal (Applications/Utilities/Terminal) and then once it’s opened type

yes > /dev/null

and hit return.

That should max out 1x core. Then open a new terminal window (CMD+N) and again type

yes > /dev/null

and hit return. Again this will max out another core

Keep on repeating this process until all your cores are maxed out.


To keep an eye on CPU usage open Activity Monitor (Applications/Utilities/Activity Monitor) and look at your CPU history to see if they are all maxed out.

You can then leave this running for as long as necessary in order to run your CPU at full load and monitor temperatures and fan speeds.

Monitor CPU core temperatures

Viewing your CPU A and CPU B temps using iStat Pro widget or similar is not that accurate or useful as it’s reading the temperature from the heatsink not the actual cores of the CPU. if you want the most accurate temperatures you need to use an app that shows individual core temperatures. I use Temperature Monitor for OS X which is free and even has history windows so you can view the core temperatures over a period of hours or days.

NOTE – To cancel the stress tests on your CPU simply close all the open terminal windows (CMD+W) and/or  quit Terminal (CMD+Q).

2 Replies to “Stress test your CPU on Apple OS X and monitor core CPU temperatures”

  1. You can do it even simpler by using

    yes > /dev/null &

    then just press up and return to send the process into the background again and again.

    When you’re sick of it. close the terminal

  2. I’ve just installed a pair of E5345 in my MacPro 1,1 and just wanted to say thanks for the information I’ve found on your site. The 2,1 firmware upgrade was a lifesaver!

    On a related note, I’m wondering if you are getting good thermal contact between your CPUs and the heatsink. From your screenshot above, it looks like you are seeing core temps of 60 degrees C, with an ambient air temp. of 22 degrees C.

    Right now, my machine is maxed out doing h264 encoding, and the highest temperature I’ve seen is 50 degrees C, with an average of 46. This is with ambient air temp of 21.

    I used Arctic Silver 5, and followed these instructions:

    The method was completely different to any I’d used before, and definitely seems to have worked well for me. My temps are still dropping, as the Arctic Silver ‘aligns’ itself over a number of hot/cold cycles.

Leave a ReplyCancel reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.