Who knew there was a “right” way and a “wrong” way to connect devices to a MacBook with Thunderbolt/USB-C ports on both sides. Do it wrong and your computer’s performance may be botched.
I recently encountered this knowledge on a thread over at StackExchange, and I was surprised to find that there can be a substantial difference in your system’s stability depending on how you’re charging and connecting devices to your MacBook—a MacBook Pro, in this example. The decision to connect everything on one side might impact how hot your system runs—and a hotter system is more likely to see its CPU throttled to reduce internal temperatures, costing you performance.
Before I get into this, I want to note that I said which side you choose to charge on can make a difference; I’m not convinced it is a universal issue for all MacBooks, nor is it the only issue that could cause your system to slow down. It’s a quirky bit of troubleshooting that most people would never think of, though, which is why it’s worth knowing about.
First, let’s consider the initial problem that led to this discovery, just so we’re all on the same page. As StackExchange user Adam posted:
Occasionally my machine will have a
kernel_taskinstance 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_taskpops up again until it finishes whatever it’s doing.
How can I find out what this process is doing?
StackExchange user BMike answered with a wonderful description of how having a number of devices plugged into the left side of one’s MacBook Pro can impact your system’s internal temperature—mainly if you have devices plugged in and are charging your MacBook Pro from that same side. You can click through to the answer to see the various graphs BMike created to track CPU use in a variety of different “what is plugged in where” scenarios, but here’s the overview:
Actual CPU temperature or application CPU usage is uncorrelated with
kernel_task. A hot CPU is throttled by reducing its clock speed, not by scheduling fake no-op load.
The graphs below are from iStatMenus. The machine had been used on battery then plugged in.
State A a USB-C hub (a mouse and keyboard, plus power) and a USB-C HDMI 2.0 adaptor, both on the left side. You can see the Thunderbolt Left Proximity temperature sensor rise quickly. About 3-4 minutes later the dreaded
kernel_taskhigh CPU usage starts.
State B cures the
kernel_taskproblem by moving power from the left ports to the right. The left side temperature drops and the
kernel_taskgoes away within about 15 seconds.
This is causal. Moving power back to the left side, restoring State A, quickly restores the temperatures and
kernel_taskagain comes back after 3-4 minutes. Again moving power back to the right side, restoring State B, resolves the problem immediately.
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 adaptor individually raise the temperature about 10 degrees, and 15 degrees together.
So, what’s the takeaway? If you’re finding that your MacBook is grinding to a halt, check Activity Monitor to see if you can find an obvious cause. If you notice a process that’s not very descriptive tying up your system’s resources—like a “kernel_task”—take a moment to look at what you’ve got plugged into your laptop, and where.
If you’re using all of the ports on the left-hand side for charging your MacBook and connecting other devices, try splitting the load: Moving your charging cable over to the right side, for example, might be all you need to do to reduce the performance throttling you’re experiencing. Similarly, resist the urge to just move everything into the right side—you might not experience the same throttling, but you could suffer some system instabilities nonetheless.
In short: Whenever possible, split the difference. And if you can’t—because you simply must have that much gear plugged into your MacBook Pro, investigate alternate cooling scenarios. Though dorky-looking, a third-party cooling pad might be all you need to keep your warmer MacBook running smoothly.