Thursday, April 23, 2020

Throttling Due to Thunderbolt Left Proximity Sensor

Dion Almaer:

Do you charge your Macbook on the left or right side? Turns out right is better!


Occasionally my machine will have a kernel_task instance max out the CPU[…] This can last from minutes to sometimes hours. The machine is effectively unusable in this state. Restarting doesn’t help; a new kernel_task pops up again until it finishes whatever it’s doing.


State C shows that simply having stuff plugged in to TB ports raises their temperature significantly. Both the hub (mouse and keyboard ONLY) and HDMI adapter individually raise the temperature about 10 degrees, and 15 degrees together.

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.

Joseph Pierini:

Just had this problem on Catalina 10.15.4.

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

Update (2020-04-23): See also: Hacker News, Igor Kromin.

Update (2020-04-24): Peter Steinberger:

This finally explains what high kernel_task is, and why it happens. I spent so much time on it; wrote radars, escalated to Apple support; replaced hardware (!) to finally see that it’s temperature emergency code because I plugged in the LG 5k monitor left.

It’s maddening. Probably a design compromise [so] it can be 1mm thinner.

It changed the way I work - almost stopped using external screens because of this bug. The back pain this caused...

Update (2020-06-05): Apple (via Lloyd Chambers):

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.

See also: Howard Oakley.

10 Comments RSS · Twitter

“Turns out right is better!”

Tell that to the people who only have charging ports on the left side (and I’m not talking about MagSafe).

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

I hope the 16" was only to bring the fix of keyboard and a new completely redesigned MacBook Lineup is still in the work. That is from 16", 14"and the MacBook Air. The MacBook Air is running with a 12W CPU max that will go up to 95C when maxed out with ZERO active cooling.

Basically we have ( and has been for a long time ) entered in the world of TDP design. We are fundamentally limited by heat dissipation, and yet every Mac Apple tweaks or design Apple has seemingly ignored it.

This… is… 

I can't even. ¯\_(ツ)_/¯

Why thunderbolt 3 makes everything so hot? I remember on my old Macbook Air when I connected to an external display the thunderbolt 2 -> HDMI dongle it wasn't even warm. Nowadays when I connect HDMI cable to thuderbolt/usb-c dongle it immediately becomes very warm to touch. And even connecting usb mouse is keyboard via usb hub, you can feel how the temperature raises. That was never a problem on traditional usb3/thuderbolt-2 ports, at least in my experience.

I have an old 2012 13" MBP that I booted up recently. It's got an Ivy Bridge CPU, and is thick because of the optical drive. One thing I noticed immediately was how rarely the fan spun up... CPU usage had to be pretty high and sustained for quite a while before I could hear the fan.

When a family member upgraded from this Mac to a 2019 Air, one of the first things they remarked upon was how frequently the fan spun up, how loud it was, and how hot the laptop could get. Thinness and weight aren't the only design aspects a laptop needs to consider. 'Design isn't just about how it looks', etc.

[…] MacBook Pro Exhibits Different Problems Depending on Which Side’s Thunderbolt Ports Are Used for C… […]

FWIW - I have a max spec 16” MBP, and I have it plugged into two 5k LG monitors regularly. The temperature regulation has been fine, although the fans spool up when I’m loading up the machine with other tasks. It’s the fact it can’t remember which monitor is which upon wake from sleep (and therefore, what rotation they should be) that’s still quite annoying...

Anyone else remember the MacBooks and MacBook Pros that had USB power problems? One of the ports was connected to a bus that seems to be sharing internal components so could not always provide enough juice to external peripherals and the other(s) was (were) not. I believe with the dual USB 2.0 MacBooks on the left side, the proper port to use was the rear one. On the MacBook Pros with ports on the left and right, you could use the left ports, but not the right one.

Warning, direct link to PDF, but Hercules Audio agrees in this support document

My MBP16 has the same issue for past 2 months. Then saw this tweet
I had the same issue where the CPU throttles even under base clock(much worse when I plug an external monitor)
Then I decided to clean the Fans and now no more CPU throttling! unless it gets to 100C.

I have a 2017 13" MBP and recently added a second external monitor via a Thunderbolt hub.

Wow what a difference it has made swapping to the right Thunderbolt ports, the throttling has instantly improved!

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