MacBook Pro Exhibits Different Problems Depending on Which Side’s Thunderbolt Ports Are Used for Charging

David Murphy, Lifehacker:

Yes, you read that headline correctly. There is apparently a “right” way and a “wrong” way to connect devices to a MacBook with Thunderbolt/USB-C ports on both sides. Doing it wrong can affect your computer’s performance.

Adam on Stack Exchange:

High kernel_task CPU Usage is due to high chassis temperature caused by charging. In particular Left Thunderbolt port usage.

Solutions include:

  • Move charging from the left to the right side. If you have a second charger then plug it in on the right side. Avoid plugging everything on the right side (see last paragraph below).


Note that high temperature on the right side appears to be ignored by the OS. Plugging everything into the two right ports instead of the left raised the Right temperatures to over 100 degrees, without the fans coming on. No kernel_task either, but the machine becomes unusable from something throttling.

Apple support:

One of the functions of kernel_task is to help manage CPU temperature by making the CPU less available to processes that are using it intensely. In other words, kernel_task responds to conditions that cause your CPU to become too hot, even if your Mac doesn’t feel hot to you. It does not itself cause those conditions. When the CPU temperature decreases, kernel_task automatically reduces its activity.

Michael Tsai:

Again, it seems like Apple’s notebooks aren’t designed with enough thermal headroom.

It seems like there are two distinct bugs occurring here:

  1. For some reason, using the left-side Thunderbolt ports to charge a MacBook Pro will cause the CPU to run hot, triggering kernel_task to also use a lot of CPU cycles, which speeds up the fans and makes the Mac unusably slow.

  2. Charging using the right-side Thunderbolt ports also makes the Mac run hot, but kernel_task does not take over the CPU and fans are not running at high speed — which they normally would be at this kind of internal temperature. This means that the CPU’s performance will be automatically throttled to guard against overheating.

Is this a thermal headroom problem? Apple tends to operate within fairly tight tolerances in their notebooks; I remember my mid-2007 15-inch MacBook Pro would frequently drift above 100°C while doing CPU-intensive tasks. High temperatures do not appear to be new, but performance degradation while charging is something I’ve never heard of before.

Update: Howard Oakley:

Your Mac’s thermal sensors are monitored by a sub-system named Core Duet, which together with the SMC manages its internal environment and services. When a process runs away and takes over the cores, or any of the internal thermal sensors registers an abnormally warm temperature, Core Duet responds with a set of actions to try to keep it cool. These include its active cooling system of fans, which are run up to speed to blow cooling air over the most sensitive components, and easing off CPU load. Where possible that slows processor speed, and blocks any runaway processes by occupying the CPU with kernel_task.

So kernel_task’s high load isn’t a cause of the problem, it’s part of the normal cooling response.

It’s problematic that charging a MacBook Pro from the left side — particularly while using other accessories on the same side — warms it up enough to trigger kernel_task. I think it’s even more concerning that using the right-hand ports for charging will warm it equally but kernel_task won’t be triggered, thereby failing to adequately cool the system.